bugfix: force bitrate to 16 for SSML <audio>.

Espeak-ng uses bitrate 16 internally. If the input inside <audio> has a
different bitrate the audio will not play correctly unless converted to
16 bits.

Closes #885.

src/libespeak-ng/readclause.c

 			strcpy(fname_temp, tmpnam(NULL));
 #endif
 
-			sprintf(command, "sox \"%s\" -r %d -c1 -t wav %s\n", fname, samplerate, fname_temp);
+			sprintf(command, "sox \"%s\" -r %d -c1 -b 16 -t wav %s\n", fname, samplerate, fname_temp);
 			if (system(command) == 0)
 				fname = fname_temp;
 		}