We have a new deployment with a Cisco 4402 and 1131 AP's. The phones are 7921's and the network consists of a 3560 (network routing device), and 2960's as the access devices. The wireless phones are on their own VLAN as are the wired phones. We are using Berbee's push to talk application server which resides on on VLAN 200 (wired phones vlan). All VLAN's have sparse-dense mode running on the interfaces and the wireless controller has multicast enabled (igmp disabled on the WLC). The push to talk application works but there are quality issues going from wireless phone to wireless phone and wireless phone to wired phone.
The issue appears to be the wireless controller (running 5.2.196). The audio from the 7921 push to talk app sounds really choppy on the receiving side. All voice calls between 7921's and any phone work great, no issues at all. The issue is just the 7921 using push to talk.
There is no traffic on the wired or wireless network today so I highly doubt this is a QoS issue.
You can also keep your 5.2 code, and set only one mandatory data rate (the lowest supported rate). This makes that the highest mandatory data rate is the same as the lowest mandatory data rate and everyone is happy...