I'm trying to capture audio from a phone on a PC sound card. What is the best volume setting for the phone to get the best quality? If the app on the phone that's producing the sound has its own volume setting, what should it be?
Gain-staging without meters…
Turn your output device [in this case your phone] up gradually, turning down your listening levels as you do this to keep the sound comfortable.
If you start to hear distortion from the loudest signals, turn the output device back down by maybe 20%.
Repeat for each stage in your signal chain.
This will drive every stage at 'maximum before distortion' with a sensible bit of headroom.