If you can get it:
Zölzer, Udo (1997). Digital Audio Signal Processing. John Wiley and Sons. ISBN 0-471-97226-6
might have to buy them used.
is your interest music processing and synthesis?
there are some nice online books. Look for anything Julius Smith. But there are others, but i can't remember any names to search for.
Many years ago, when I was studying audio DSP in my university, I had to read the following:
Y. You, “Audio Coding: Theory and Applications,” Springer, 2010 here
A. Spanias, T. Painter, V. Atti, “Audio Signal Processing and Coding,” Wiley, 2007 here.
I was very dissapointed by both. Spanias & Painter provide a nice and thorough overview of MP3 coding (...
My personal favorite, though I don't usually do much audio signal processing, are Julius O. Smith III's Spectral Audio Signal Processing and Physical Audio Signal Processing. They're available online here and here.
A while ago, he made the Physical book available in paperback on Amazon.
I am not sure there are "best" image processing or computer vision books to buy. The topics covered can be very wide, and some can be better on some aspect (morphology, segmentation, denoising, etc.).
Some sites recommend a hanful of such books, and link to their draft versions, as in 8 Books for Getting Started With Computer Vision. Indeed, a lot of ...