iPhone 16 speaker experiences lag and looping sounds

There may be several reasons due to which an iPhone may behave as described by you. Some of them could be...

Let's delve a bit deeper into some of them:

1. Storage: A full iPhone can definitely slow things down. Phones can experience performance issues when the storage is nearly full. Users can check their storage status by going to Settings > General > [Device] Storage. Deleting unnecessary apps, photos, or videos can free up space and improve performance. How to check the storage on your iPhone and iPad - Apple Support.
2. Battery Health: A degraded battery can impact performance. A deteriorating battery can impact the overall performance of an iPhone. Checking the battery health can be done by going to Settings > Battery > Battery Health. If the maximum capacity is significantly reduced, it might be time to replace the battery. Prolong and Maximise Battery Performance … - Apple Community
3. Incorrect Handling: Accidental drops or bumps can cause internal damage. looking at the Apple's guide on proper handling is a good step. Important handling information for iPhone - Apple Support (IN)
4. Environmental Issues: Extreme temperatures, dust, and water exposure can all be detrimental.
   1. Keeping iPhone clean is an important step. Dust accumulation in the iPhone's ports and openings can affect functionality. Apple provides guidelines on how to clean iPhones safely, which usually involves using a soft, lint-free cloth to remove dust without causing damage. Cleaning your iPhone - Apple Support
   2. Keep the iPhone within temperature limits. Phones are designed to operate within a specific temperature range. Exposure to extreme temperatures, especially in parked cars, can lead to overheating or other performance issues. If your iPhone or iPad gets too hot or too cold - Apple Support.
   3. Addressing safety issues is also important: Important safety information for iPhone - Apple Support (IN)
5. Unauthorized Service: Servicing an iPhone through unauthorized channels can result in substandard repairs and void warranties. Users should always seek authorized Apple service centers for repairs and maintenance.
   1. Repairs by unqualified technicians can introduce new problems. Find information about your warranty or AppleCare plan - Apple Support is helpful.
   2. Modification / Alteration of an iPhone by a third-party vendor may cause issues beyond repair. --> Unauthorized modification of iOS - Apple Support (IN)
6. Jailbreak: Now, let's talk about jailbreaking. Jailbreaking removes Apple's restrictions, allowing for more customization and app installation outside the App Store. However, it can also lead to instability, security vulnerabilities, and issues receiving future iOS updates.

By considering all these possibilities, you've provided a comprehensive approach to troubleshooting the iPhone's behavior.

Understanding and addressing these factors can help you maintain the optimal performance and longevity of their iPhones. Following Apple's guidelines and seeking authorized support when needed are crucial for a positive user experience.

If you are sure it is the fault of the equipment.

If you have either dropped the device accidentally or exposed it to liquid or used it beyond acceptable temperature limits then…

Take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support:

Find Locations - Location Selector - Apple

Contact Apple for support and service - Apple Support 

Get a Genius Bar Reservation and Apple Support Options - Apple

Contact Genius Bar.

Manage Reservations - My Reservations - Apple

When an iPhone is connected to a car’s audio system via Bluetooth, it will not automatically switch to another Bluetooth device unless specific conditions are met. These are:

1. The current Bluetooth connection is interrupted or lost, typically due to signal interference from nearby electronic devices, physical obstructions, or the car's Bluetooth system momentarily dropping the connection. However, this interference usually does not come from another Bluetooth audio device actively trying to take over.
2. Once the iPhone is disconnected from the car’s Bluetooth, it may automatically connect to another previously paired Bluetooth device—but only if:
   1. That device is powered on, within range, and set to allow automatic connections.
   2. It was already paired and trusted by the iPhone in the past.
   3. The iPhone has not prioritized another known device for automatic connection (Apple devices tend to remember recent usage patterns).

In short, the iPhone won’t jump from one active Bluetooth audio device to another unless it gets dropped by the first and the second was already on friendly terms.

You may refer: --> How to Stop AirPods Switching to Other Devices Automatically

1. Make sure your AirPods or ‌AirPods Pro‌ are connected to your ‌iPhone‌.
2. Launch the Settings app.
3. Tap Bluetooth.
4. Tap the encircled info (i) button next to your AirPods in the list.
5. Tap Connect to This iPhone.
6. Select When Last Connected to This iPhone.

See the Pic below.

Bluetooth interference can potentially cause connectivity issues due to the disruption of the wireless communication between devices. Bluetooth operates in the 2.4 GHz frequency range, which is also used by various other devices such as Wi-Fi routers, microwaves, cordless phones, and even some baby monitors. When multiple devices are operating nearby and using the same frequency band, interference can occur.

Here's how Bluetooth interference can impact connectivity:

1. Audio Streaming Interruptions: When you're streaming music via Apple Music or while playing audio content, Bluetooth interference can disrupt the connection between your device (such as an iPhone) and the Bluetooth speaker or headphones. This interference can result in the audio stuttering, dropping out momentarily, or experiencing poor sound quality.
2. Bluetooth devices transmit data in packets. Interference can cause some of these packets to be lost or corrupted during transmission. As a result, the receiving device may have trouble decoding the audio correctly, leading to stuttering or delayed responses.
3. Bluetooth signals can be weakened by interference from other devices operating on the same frequency band. If the signal strength drops significantly, it can affect the stability of the connection between your device and the Bluetooth accessory. This can result in intermittent stuttering as the connection struggles to maintain a consistent data flow.
4. Bluetooth uses different channels within the 2.4 GHz frequency range to establish connections. When there are many Bluetooth devices nearby, multiple connections can create congestion on the available channels. This congestion can cause interference, leading to degraded performance for Apple Music streaming.