How do I fix uncontrollable volume fluctuations on iPhone 12?

You may try some of these steps, please check after each step...

1. Remove any protective films or cases that might block/activate the button or switch. · Check for debris or dirt around the button or switch and clean with ... If a button or switch isn't working on your iPhone or iPad
2. See the pic below which shows what I have observed and is self-explanatory.
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3. Try change with buttons [Disabled]
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4. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
5. Reset Network Settings: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings: Only network settings are removed.
   1. When you reset network settings, previously used networks and VPN settings that weren’t installed by a configuration profile or mobile device management (MDM) are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and Ask to Join Networks settings remain turned on.
   2. To remove VPN settings installed by a configuration profile, go to Settings > General > Profiles & Device Management, select the configuration profile, then tap Remove Profile. This also removes other settings and accounts provided by the profile. See Install or remove configuration profiles on iPhone in this guide.
   3. To remove network settings installed by MDM, go to Settings > General > Profiles & Device Management, select the management, then tap Remove Management. This also removes other settings and certificates provided by MDM. See “Mobile device management (MDM)” in the iOS Deployment Reference.
   4. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
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6. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
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7. Make sure you have the latest updates installed.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.

On an iPhone, Media Volume, Alarm Volume, Alert/Ringer Volume, and Call Volume are related but not all controlled by the same setting.

1. Media Volume

This controls the sound produced by apps and media, such as:

YouTube and other videos

Music

Games

Podcasts

Most app sounds

Audio played through the iPhone speaker, headphones, CarPlay, Bluetooth, etc.

How to control it:

Press the Volume Up/Down buttons while media is playing, or

Open Control Center and move the volume slider.

The volume buttons normally control media volume when you are actively playing media.

2. Ringer / Alert Volume

This controls sounds such as:

Incoming phone calls

Text/message notifications

Mail notifications

App notification sounds

Calendar/reminder alerts

Other system alert sounds

Go to: Settings → Sounds & Haptics → Ringtone and Alerts

The Ringtone and Alerts slider controls this group of sounds.

You can also enable Change with Buttons. When enabled, the physical volume buttons can change the ringtone/alert volume when you're not actively controlling media.

Important: Turning media volume down to zero does not necessarily turn notification/ringer volume down to zero.

3. Alarm Volume

This is where there is an important distinction.

For Apple's Clock app alarms, the alarm volume is generally tied to the Ringtone and Alerts volume.

So if you set:

Settings → Sounds & Haptics → Ringtone and Alerts → 50%

your normal Clock alarm will use that level.

Changing the volume of a YouTube video, for example, doesn't change the Clock alarm volume.

Also, the Silent Mode switch does not silence a normal Clock alarm. That's intentional—otherwise it would be rather easy to sleep through an alarm.

4. Call Volume

There are actually two different "call volumes."

A. Volume of the person you're talking to

During a phone call, the side volume buttons control the volume of the other person's voice coming through the earpiece or speaker.

This is separate from your normal media volume.

For example:

You are listening to YouTube → volume at 70%

You make a phone call → you set call volume to 40%

Changing the call volume doesn't necessarily change your YouTube volume.

B. Ringtone volume

Before you answer a call, the incoming ringtone uses the Ringtone and Alerts volume described above.

Once you answer, the volume buttons control the call audio.

One particularly important iPhone setting

Go to: Settings → Sounds & Haptics → Ringtone and Alerts

You'll see the Ringtone and Alerts slider.

Below/around it you'll also find Change with Buttons.

If Change with Buttons is OFF, the physical volume buttons generally won't accidentally change your ringtone/alert volume when you're not playing media.

If Change with Buttons is ON, pressing the volume buttons when you're not actively playing media can change the ringtone/alert level.

So, if you want your alarm and notifications to remain at a fixed level, while freely using the side buttons for YouTube/music, keeping Change with Buttons OFF is usually the better arrangement.