Why does my volume keep going down

First remove the case if you use any this might be the problem. Try clearing your system cache and restarting your device. If that doesn't work try putting your samsung into safe mode. If that fails it could be a problem with your volume button sticking

Clean physical buttons:

Sometimes, physical buttons can get stuck or malfunction. Inspect your volume buttons to ensure they are not pressed or stuck.

If you notice any issues, gently clean the buttons (don't use running water) and see if that resolves the problem.

Restart in safe mode:

1. Hold down the Power button for a couple of seconds.
2. Choose Restart from the options that appear.
3. If your phone vibrates, press the Volume Down button until you see the lock screen.
4. Release the Volume Down key if Safe Mode pops up.

Links: Video how to put SAMSUNG M51 in safe mode

Whipe chache partition:

1. Turn off your device.
2. Press and hold the Power key beyond the model name screen.
3. When “SAMSUNG” appears on the screen, release the Power key.
4. Immediately after releasing the Power key, press and hold the Volume down.
5. Press the Volume down key several times to highlight “wipe cache partition.”
6. Press the Power key to select.
7. Highlight “yes” and press the Power key to confirm. Once the wipe cache partition is complete, select “Reboot system now” to restart your device
8. Links: How to wipe cache

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Software Update:

There might be a software update, update your phone and see if this solves it.

If noone of these steps works there might be a bug or some advanced setting, it can also be because of damage.

There might be the Media Volume Limit that is activated so you find how you turn off media volume limit by clicking here (external website). But the screen shouldn't show that volumes goes down and it should only occur when using headphones.

I also have an nice app you can get you a log of the applications that uses the sound nice catch on galaxy store

it might help you to troubleshoot.

Give these steps a try, and hopefully, your volume issue will be fixed!