A lot of it applies to software defined radio processing as well, other than tending to work in real vs complex, but you can always do either.

For any one-dimensional signal, honestly.

Audio is just the most common use case.

Vast majority of this book covers DSP in very broad generality, much akin to what you would see in an undergrad EE course on the topic. Compare with Oppenheim and Schafer. Different focus but much of the same content.

Without loss of generality.