Hmm..
Perhaps the soundfont is defect(?)
Could you try a different GeneralUser?
We recommend http://www.schristiancollins.com/generaluser.php
Besides that, since you are new in Forum
Welcome eyesis !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480
Let us know the outcome