Actually, I may have just solved your issue. Out of curiosity, I tried playing music through the speaker on my phone (which is separate from the mic array), and I got an empty channel 5, the same as you.
I think that in order for this to work as expected, the music must be playing through the 3.5mm jack on the mic array board.
You’ll want to select the mic array as Audacity’s playback device and connect an external speaker to the mic array via the 3.5mm jack.