'Sounding weird' says a lot, but isn't very descriptive. 320 is the best resolution you'll get from an MP3. I'd say most people couldn't tell the difference between 320 and a 24bit/44.1khz wav file. Any lower bitrate MP3 will sound worse than 320.
I notice a difference from the master bus mix and from an exported file. I think Steinberg does something on the master bus to bump up the volume or do some kind of background limiter function.
Isn't Cubase 5 restricted to 20 MP3's? Did you have to buy an MP3 upgrade patch or something? I export out from Cubase to a wav file and then convert to MP3 in wavelab. I could never get the MP3 export to work from Cubase. I think you have to buy something.
Yeah, I always thought it kinda lame too. A silly incentive to buy a full version. I never got credit for purchasing after upgrade from LE. Must be a German thing. Wait, I'm of German descent...Hmm...
try converting using the franhoufer codec, set it for highest quality, 320kbps also make sure the output is no louder -0.5dBFS on the peaks and it might sound less weird.