sound: Use M_WAITOK where possible

These malloc(9) calls are in contexts where sleeping is permitted.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, zlei, markj, emaste
Differential Revision: https://reviews.freebsd.org/D46845

(cherry picked from commit 5e33eca8e8f359d4d48f4c50334a03748420a7da)