It's been said that you need to open a separate window to transpose an audio file. This is not so. If you press the +, or - transpose button, the screen shows an icon labeled one of three choices:
MASTER
KEYBOARD
AUDIO
You can simply touch the icon, and select what it should change, and it will stay that way ... at least until you turn off the kb. Haven't gone past that step so far.

For me, I like it set to MASTER. That way, everything changes - if I choose to give my voice a break, or sweeten up a sound with a key change.