To reduce latency, AudioRelay sends the audio data on your device's "fast path". But on some devices, this results in a quality loss. (Confirmed on Xiaomi phones)
To fix this, try the AudioTrack output in the Android app settings. Note that this could increase the audio latency.
Inside the Android app, go to settings
Uncheck low latency
Otherwise, check that Windows is emitting the sound to a format of at least 48kHz
Type sound settings in the Windows search bar
Click on Additional properties at the bottom
Change the default format
Why is the volume so low?
The audio's volume received by AudioRelay can sometimes be tied to the volume of your PC.
Try to increase the volume in Windows before muting your PC.
There's a lot of latency when listening through a Bluetooth device
Unfortunately, audio played over Bluetooth is inherently going to have latency. You can observe the delay of your headset by playing with a soundboard app, or using this one.
If you don't notice any latency when using YouTube, it's because videos apps can compensate for the lag by synchronizing both the image and the audio. You can still notice the audio delay by pausing and resuming a video.
How to reduce lags and packet loss?
If you don't need Bluetooth, deactivate it. It can cause interferences