My understanding is that you can connect the audio source's ground to the +2.5v level obtained from the voltage divider.
Probably not a good idea, as "both" Arduino and audio circuit are likely grounded
to the same point eventually - ie, mains ground. ARduino will get it via plugging
in the USB port, and audio amps are just made that way.
Even if someone says they're only gonna use the ckt with their battery operated
dowhatjamacallit, one day they or someone else will plug their AC powered audio
amp into the ckt.