Yes, you would have to purchase the second phone and then transfer the service over, before returning the original. Good thing you realized this before your friend did the return!
As far as the audio quality, it may depend on the area and congestion of the network. I've had an AT&T TracFone for ages (my main postpaid phone is on T-Mobile) and sound quality is always below-par compared to my T-Mobile phone, which sounds _very_ good. Does your friend know of anyone else on AT&T that he can ask or compare to? I find sound quality on T-Mobile, Sprint, and Nextel to be very good. AT&T and Verizon are a little muted and just not as rich sounding, and I'm talking about several different phones not just one. A lot of it is subjective so YMMV.
Castrol EDGE 5W-30 is my drug of choice.
Primary: iPhone 6s Plus 64GB, Galaxy Note 5, LG G5: 5 lines, 3x $5 Android web, 1x $2.99 T-Zones
Secondary: TracFones: iPhone 5s [AT&T] & iPhone 6 [Verizon]
Mobile Broadband: ZTE Z915 LTE hotspot (5GB for $10) + iPad Mini Retina w/ T-Mobile LTE