Hi, brandystarbrite said right, i also have this problem with sound quality.
Sometimes it might help to use for each track EQ(equalizer) you can cut a little some high or low frequencies. Or use limiter, or gate, or some settings.
Actually there is a company Cakewalk, which made very nice VSTi sfz+ plugin for DAW software, it can help you very much with sound quality problems, here is the link for it:
http://www.cakewalk.com/products/sfz/
First you have to register on that website, and than download the plugin for free. Than install it to a folder "VSTplugins" for LMMS. It sounds much better than build-in sf2 sampler, in my opinion.,
Also you can get a very good VSTi plugins producing a great sound, and they a all free, here is the link where you can download it:
http://www.vst4free.com/
And there is one more nice plugin, but i don't know yet how to install it to LMMS, haven't try it. Here is the video where you can hear how it sounds:
http://www.youtube.com/watch?feature=pl ... Wfqcf-EyUE
Sometimes i could get a pretty good sound from LMMS, but i have never got a studio sound quality like in commercial music(
Anyway you sure need to do mastering for your music, when you finish it. Use enhancer and maximizer at least.
Anyway hope what i said will help you.