Are you sure you didn't play the channel twice? Debug the play and stop channel values.
// ------------------------------------------------------------- //
// PLAYSOUND Sample
// ------------------------------------------------------------- //
SYSTEMPOINTER TRUE
// Laden eines Sounds aus einer Wav-Datei / Load sound from wav-file
LOADSOUND "sensaura.wav", 5, 4 // Num: 0 - 127 max
LOCAL iSoundChannel% = -1
LOCAL bPlaying% = FALSE
WHILE TRUE
MOUSESTATE mx, my, b1, b2
IF my>100 THEN my=100
SETMOUSE mx, my
mx=INTEGER((mx-320)*10/32) // -100 <-> +100
// PAN
DRAWLINE 0, 100, 200, 100, RGB(255,255,255)
DRAWLINE mx+100, 90, mx+100, 110, RGB(200,255,255)
// VOL
DRAWLINE 0, 130, 200, 130, RGB(255,255,255)
DRAWLINE my*2, 120, my*2, 140, RGB(255,255,200)
IF b1 AND bPlaying=FALSE THEN iSoundChannel% = PLAYSOUND (5, mx/100, my/100)
IF b2 THEN STOPSOUND iSoundChannel%
bPlaying% = SOUNDPLAYING(iSoundChannel%)
PRINT "Pan[%]:" + mx + " Vol[%]:" + my, 84, 150
IF bPlaying% THEN PRINT "Is playing!", 84, 180
SHOWSCREEN
WEND
HUSH // Ruhe / Silence
This works pretty fine for me.