A search for open caveats in the Cisco controller release notes for version 18.104.22.168 revealed an interesting finding...
Looking up the bug on the Cisco bug toolkit reveals an issue with Low Latency MAC operation with 802.11n access points. This specific bug requests that Low Latency MAC should have no effect on 11n APs until the original bug is fixed. This was implemented as of currently in-development code version 22.214.171.124, and is really only a workaround.
CSCtc73527 Bug Details
The original bug details the issue with 11n APs failing to properly downshift to lower voice data rates (also referred to as 'nominal' data rates in Autonomous software) when attempting frame re-transmission. Note that this bug is still open at this time.
CSCsy66246 Bug Details
To briefly explain the issue, Low Latency MAC is a feature which limits the attempted number of packet retransmissions to 3 attempts for packets in the voice queue to age them out quickly. Voice is a real-time protocol and frames become useless very quickly if delayed too long. They can disrupt a voice conversation and actually be counter-productive. It also controls the nominal data rates used to transmit packets in the voice queue, thereby reducing data rate shifting to a minimal set of data rates to ensure successful delivery of packets and avoid retransmissions.
So rather than continue to repeatedly attempt frame re-transmission, the network will only try 3 times at which point it will drop the frame. This will help clear up network and access point resources to deliver frames which still have validity for a voice call.
If you're deploying voice on 11n APs, be sure to disable Low Latency MAC... for now (and keep a close eye on the status of this bug ID). You can still safely enable EDCA, which controls the Adaptive Inter-Frame Spacing and Contention Window values in a QoS Basic Service Set, to any of the available settings without issue (WMM, Spectralink, Voice Optimized, Voice and Video).