Thanks for the suggestions, lizzi555. QoS was indeed activated. Here are the tests that I just tried:
QoS, WMM, IGMP snoop all on: no joy
Qos OFF, WMM on, IGMP snoop on: no joy
QoS OFF, WMM OFF, IGMP snoop on: no joy
QoS OFF, WMM on, IGMP snoop OFF: no joy
QoS OFF, WMM OFF, IGMP snoop OFF: no joy
WMM and IGMP snoop are AP-side only settings. QoS was on on both sides, turned it off on both.
Is your configuration similar to mine? That is, 1522-AP, 1522-bridge, and a music device off the bridge?
Thanks