Didn't notice a difference between snd_async_minsize 0;snd_async_fullyasync 1 and setting snd_async_minsize to either 262144 or 524288. The severe stuttering only happened with snd_async_minsize 131072.
The game does randomly freeze for a second or two every now and then though, but it doesn't seem to have any correlation with sounds playing at the same time.