TuxGuitar is amazing, and its open source! However, in Ubuntu the default soundbank doesn't sound as good as what is available out there. Let's make it sound better with more realistic sound (a.k.a. Real Sound Engine)...
In a terminal window, use these two commands to install the Fluid Soundfonts:
sudo apt-get install fluid-soundfont-gm
sudo apt-get install fluid-soundfont-gs
If these terminal commands don't work, then you can use the Synaptic Package Manager to install them.