Open app: a sound. Start/Stop recording: a different sound. Start playback: another sound. Swap apps and return to Audition: oh hey, a notification sound.
It's only in Audition, no other Adobe app nor any other app in Windows. It does this whether I change settings in Prefs>Audio Hardware/Mapping from ASIO UA to MME Default (UA USB). I have no other audio drivers installed. I have Systems Sounds (Asterisk, Start, Notifications, etc) disabled. It did not do this before installing/using the Apollo interface - I previously was using a Behringer 302USB mini mixer for my XLR Mic input and did not have this issue in Audition.
Well I know what it is - but I'm afraid I don't know exactly why your interface is doing it!
The moment I heard the piano sound, the first thing that jumped into my head was MIDI. What is happening is that Audition can input and output controller commands, and these are transmitted and recieved as MIDI notes. For whatever reason, your interface is treating these as something it should play! We've come across it before, but not with a UA Apollo twin. This is almost certainly an input setting on