The XMPlay.ini entry MixerPos controls where the mixer dialogue appears on screen. Either set it to MixerPos=0000000000000000 to have the mixer positioned at the top left corner, or remove this entry completely. It gets recreated/updated upon each call of the mixer.
Alternatively you can rename your current XMPlay.ini (e.g. XMPlay.ini.bak) to keep your current settings and run XMPlay, which will autocreate a fresh ini file which should fix the mixer positioning issue, too. Then you can reincorporate the settings from your backup ini file.