It comes down to the nature of the data its processing.
Why video can use a lot of parallel processing is that there are thousands/millions of similar basic sets of operations to be done on all those pixels, so it is more straightforward (but still non-trivial) to design software that apportions the tasks systematically among the video cores, which are designed with that type of task in mind.
Audio streams are serial and usually use different complex processing chains for each of the relatively fewer streams, so making sense to use threads on whatever core is to hand, which is for what general purpose CPUs are designed. The main exception is, as Norbury mentions, reverbs, where there are the masses of simpler vector operations, akin to ray-tracing in video processing.