This is quite impressive. I can't attend them all but surly will get to some. It's from September 8 to October 19 and features many different performances, classical, jazz and pop. Click on each for a preview and dates/times: http://www.bangkokfestivals.com/home/
MRT Subway Station 'Thailand Cultural Center' has a free shuttle 5:30 to 7:00 from Exit 1.
These look pretty good to me:
Carmina Burana
An Evening of Jazz
The Paul Taylor Dance Company
Hollywood Sound of Cinema