#53 fix the issue pa_stop_sound() will sotp all of playing sound

[PengKe]

It can be reproduced by follow steps,

iaxc_sound *ring;
iaxc_sound *buzy;
...
iaxc_play_sound(ring, 1);
iaxc_play_sound(buzy, 1);

...
iaxc_stop_sound(buzy);

both ring and buzy will be stop,I test this patch it pass in Windows and Linux.

[PengKe]