If sox is sending you data and you're not reading it, the pipe will fill up and sox will block trying to print to the pipe.

One bug: You never call waitpid to cleanup the zombie child process.

Another bug: You pass user-provided text the shell without (usefully) validating it and without escaping it.
"sox -t $type - -s -r 8000 -w -t wav - 2>/dev/null"
should be
"sox -t \Q$type\E - -s -r 8000 -w -t wav - 2>/dev/null"

You should probably flush your pipe after writing to it (or turn on autoflushing on it).

Update: I had the sox and the parent program mixed up. Fixed.