Is there any value named SoundDevice under
HKCU\Software\Patchou\Messenger Plus! Live\your@email.com\Preferences
in Registry. Because if it is set wrongly, Plus! cannot play its sounds.
quote:
Originally written in Messenger Plus! Live- Registry Settings
Messenger Plus! uses this sound device ID when calling sound related functions to play or record sound (such as waveOutOpen). If -1, the default wave mapper is used. If 0 or superior, the specified sound device is used.
If this value exists, delete it and try now.
Also you can try
Messenger-Plus!-Tweak-Live script to change that setting easily.