Almost no hardware audio decoders use DSPs, or at least actually need to, since about a decade or more now.
The manufacturers you mention use DSPs for bad reasons - usually the inertia of software already using a coprocessor and not being willing to license a new codec for the general purpose core. Audio decoding and encoding is an extremely low percentage of available CPU, and it's just not worth off-loading it.
Video is another matter, at least when talking about HD. It's still very much worth using special purpose DSPs for that.
For audio, the existence of hardware DSPs is simply not a barrier to entry.
The manufacturers you mention use DSPs for bad reasons - usually the inertia of software already using a coprocessor and not being willing to license a new codec for the general purpose core. Audio decoding and encoding is an extremely low percentage of available CPU, and it's just not worth off-loading it.
Video is another matter, at least when talking about HD. It's still very much worth using special purpose DSPs for that.
For audio, the existence of hardware DSPs is simply not a barrier to entry.