A playing sound can't be stopped. Only music can be paused.
Playsound works like this:
playsound (soundname,volume, channel, loop)
Volume can range from 0 - 255.
Channel is a sound channel from 0-6 which means that 7 sounds can be played at the same time. Use -1 for EGSL to choose the next free sound channel.
Loop means: -1 = infinite, 0 = once, 1 = twice,...
Hope that helps.