1. Switch to the internal memory by selecting a folder like "/sdcard/voix" in Settings > Folders and Files. This is just for testing purposes. When you find the right settings, you can switch back to the external storage.
2. Select your device in Settings > Recording again (even if it's already selected).
3. Make sure that the default sample rate and channels mode are set for the selected Recording method.

1. Make sure other recorders aren't active while in-call (voice control apps, navigation apps, other call recorders, etc).
2. To ensure that recording is not started too early, either disable automatic recording and start recording manually after the other side answers, or stop it and restart after that moment. If both sides are clearly heard in the part of recording that was manually started later, then you need to increase "delay before start of recording" in Settings > Recording.
3. If the volume is low for a specific type of calls only (i.e., either outgoing or incoming ones), or for random calls, then increasing this delay should help.

Unfortunately, there's no way to fix it. The app records what the system provides.

Unfortunately, there's no general solution here. Try increasing "delay before start of recording" in Settings > Recording. Also, check if turning the speaker on then off in-call will help.