In essence, app-specific volume allows you to have the volume in Infuse lower than the system volume. With the volume in Infuse all the way up, it would match the system volume. This is how other apps like Quicktime, Spotify, etc… work as well.
If you see this again, can you hit the ‘Email Us’ option in settings and post the 5 digit code?
This is something we have consider in the future. There is a thread in the suggestions area you may consider upvoting.