I don't know what you have tried and have a different device so the menu might be different. Also change to UI 6.1.1 may have changed Go to Settings, scroll down to Sounds and Vibration, scroll to Volume., you should see four options. Increase the setting for media volume, If this works, explanation, this is the standard default volume. When using the phone on a phone call you may have been able to have it using the last setting for phone calls giving a decent call volume.
Buds: recently I started to use a bluetooth headset on my S23C FE, swapping headsets when the battery went flat, having to pair again because I use the headset on different devices. Having troubles I found that I could only change the volume on the headset itself, so worth trying volume settings on the buds if they have volume options. On the other hand, the problem solution in the paragraph above may have also solved this problem.
Tone after hanging up, I searched and found the following suggestion: go to dialer (dial key pad), go to three dots top right, scroll to Settings,scroll to Call alerts and ring tones, turn off Play tone when call ends. If it is already off try togglin it on, backing out, coming back to it and turning off again. Also in the same area ensure that Vibrate when call ends, is turned off.
