The issue of AirPods Pro suddenly getting quieter can be caused by a few different reasons. Here are some possible causes and their solutions:
Solution: Check if your AirPods Pro earbuds are clean. Earwax or other dirt secreted by the ear may clog the earbuds, affecting the sound quality. The earbuds and speaker mesh can be cleaned gently.
Solution: Check if your device has a volume limit set. On iOS, it can be checked in Settings >Volume Limits"; On other devices, look for similar settings.
Solution: Some devices automatically adjust the headphone volume based on ambient noise. Check your device settings to see if a similar feature is turned on, and try turning it off.
Solution
Try to re-pair your device after forgetting it. Find your AirPods Pro in your device's Bluetooth settings, select Forget this device, then put them back in the charging case and try to pair them again.
Make sure your AirPods Pro and paired devices are close to each other.
Solution: Check the audio balance settings. If the audio balance is adjusted to the left or right, the volume may feel lower. On iOS devices, it can be adjusted in Settings > Assistive Use > Listening > Audio Visual > Balance.
Solution: Check that your AirPods Pro and paired devices are updated to the latest version of the software. Sometimes, a software update can fix compatibility issues or other issues that cause the volume to suddenly become low.
Solution: If none of the above steps work, try resetting your AirPods Pro. Put them back in the charging case, then press and hold the setting button on the back of the case for about 15 seconds until the front indicator flashes white and then yellow, which means that the AirPods Pro have been reset.
If the problem persists, it could be that the AirPods Pro themselves are faulty. In this case, contacting Apple Support for further assistance may be the best option.