On the Sam460ex with the Radeon HD decoding some possible audio stutters with 720p videos maybe a clue . So could audio handling be a cause or just a symptom ?
Very unlikely. The audio doesn't take much processing time.
I used to think that the audio stuttering during graphics heavy stuff hogging the PCI(e) bus. However, geennaam's experience with writing the HD audio driver suggests otherwise. Most AHI drivers rely on AHI for the timing of sending new buffers to the sound card. His HD audio driver uses the HD audio hardware itself to determine when new buffers should be pulled in, which is a more accurate method. I suspect that relying on AHI's timing makes sending audio data to the sound card rather fragile. When it's slightly too late (due to heavy machine load), then stuttering occurs.
The Southern Islands UVD hardware is noticeably slower than the newer UVD in Polaris based graphics cards. The problem is most likely the way we're using it.