It sounds like you are intermitently not getting audio from your speaker on the phone. I have some troubleshooting from the following article that I would recommend:
iPhone: No sound or distorted sound from speaker
- Verify that the volume is set to a level you would normally be able to hear.
- Ensure that there is nothing plugged in to the headset jack or the dock connector.
- If the iPhone is in a protective case, make sure that the speaker port isn't blocked by the case.
- Make sure that the speaker and dock port aren't clogged with debris. If necessary, clean it with a clean, small, dry, soft-bristled brush. Carefully and gently brush away any debris.
- If an audio issue occurs when using a specific application, try testing other applications to see if the issue persists.
- If the iPhone is paired with a Bluetooth headset or car kit:
- Try turning off Bluetooth.
- If you experience difficulties with the Bluetooth feature, follow these troubleshooting steps.
- Restart the iPhone.
- If restarting doesn't fix the issue, ensure that your iPhone is updated to the latest version of iOS.
- If the issue is not resolved after restoring the iPhone software, please contact Apple Support.
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All the very best,