iOS + audio interface output volume too low

I am on iOS, connected to a Scarlett solo Focusrite audio interface via a USB to Lightning adapter. I am monitoring output with headphones plugged into the audio interface monitoring output. I can record music and it sounds good, but the app volume is so low that I have to crank up the monitoring volume on the interface to even hear myself play. With the interface volume turned all the way up, even the metronome doesn’t get very loud.

I know this is a problem with the audio output of the app, because the built in app metronome (even at full volume) does not get nearly as loud as playing songs on Apple Music while connected to the interface. This is not the case when playing through the iPhone speakers, where both apps have similar volume output.

Do you have any suggestions?

Try to enable the ‘Mic auto gain’ setting in the Settings box, this will disable the ‘measurement mode’ mode of the iOS audio session, and depending on the device might also boost the output level, because the system adds some post-processing to the audio. Alternatively or in addition to that you can also add a the Limiter plugin to the master channel to increase loudness without incurring into distortion (songs you hear on Apple Music are mastered so are typically optimized for loudness).