Are all parties connected via WiFi or is the sender using a
wired connection to the Access Point? Broadcast data will
be transmitted as unicast data from a station to an access
point and the access point will then retransmit the data
as broadcast/multicast traffic so it will use twice the
on-air bandwidth compared to when the sender sits on the
wired side of the AP.
When sending a unicast frame the AP will wait for an ACK
from the receiving station and it will retransmit the
frame until the ACK arrives (or it times out). Broadcast/multicast
frames are not ACKed and therefore not retransmitted.
If you have a busy/noisy radio environment this will
cause the likelyhood of dropped packets to increase,
potentially a lot, for multicast traffic compared to unicast
traffic. In an audio application this could certainly be audible.
Also, IIRC, broadcast/multicast traffic does not use the
RTS/CTS procedure for reserving the media which exarbates
the dropped packets problem.
It could actually be the case that multiple unicast streams
work better than a single multicast stream under less-than-ideal
radio conditions given that the aggregated bandwidth is
high enough.
If you can I would suggest that you use wireshark to sniff
the WiFi traffic and take a look at the destination address
in the 802.11 header. Then you can verify if the packets
are actually broadcast or not over the air.