Audacity doesn't have a treble control, but it does have the equalizer.
Select a short portion of sound that illustrates the error best.
Effect > Equalizer
The tool window arrives way too small to do anything useful, so grab the lower-right and pull the work window as large as you can. That blue line is a rubber band and you can click on it in multiple places and pull it around. Up is louder.
This is a very bad voice filter.
http://kozco.com/tech/audacity/voiceEq.jpg
This one's a rumble filter I designed.
http://kozco.com/tech/audacity/100Hz_Rumble.jpg
100Hz to 200Hz is bass. Below 100 is sub bass. Fingernails on blackboard happen around 3000Hz. and Sparkly Live sounds are affected by what happens above about 5000Hz. All numbers are approximate. Your Mileage May Vary,
Another example: the difference between 5000Hz and 15000Hz for the upper registers, is the difference between AM radio and FM radio.
The idea is to keep playing back the small segment after you apply different curves. It doesn't happen in real time. Bend the curve, apply it, listen to what happened. Edit > Undo, repeat
When you get one you like, Edit > Select > All and apply it to the whole performance.
We assume you're listening on a set of very high quality earphones or a terrific sound system. If not, you may get some nasty surprises, like delivering a show with MetroBus motor noise in the background because you just can't hear it on your $12 speakers.
Koz
I have a piece of audio, acoustic guitar, and I want to "sharpen" it. By sharpen, I mean that I want to add more treble, cause right now it sounds a little muffled, if that makes any sense. I REALLY appreciate any help. 
