sound: Initialize channels with sane default rate and format

There is no good reason to initialize with AFMT_U8 at 8000Hz.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48434

(cherry picked from commit e89ee05b7cf1a54d0d6ed56e4d451fdd9a10db43)