gh-96821: Fix undefined behaviour in `audioop.c` (#96923)

* gh-96821: Fix undefined behaviour in `audioop.c`

Left-shifting negative numbers is undefined behaviour.

Fortunately, multiplication works just as well, is defined behaviour,
and gets compiled to the same machine code as before by optimizing
compilers.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>