Where in Taipei is a good live music venue?
The National Concert Hall often stages recitals by world-class orchestras and ensembles.
There are occasional performances in the Muddy Basin by some Ramblers. They only happen when the saxophone player can find his walking frame though. 
There are a number of good live music venues in Taipei. What are you after, performing or attending?
For a pretty comprehensive list of what concerts are on around Taiwan, check out my site GigGuide.tw
Hopefully you can find something there.
GigGuide and some other useful sites are listed in the Taiwan Events Links over here.