On the bigger side (100+ seats), the sports center in Datong has a great auditorium. Cannot say for sure, but assume that all of the new sports centers around town must have them, as well. On the smaller side, the KHS music building on Fuxing South Rd behind the Howard Plaza has some recital rooms that are nice, but they are of the small stage/podium variety with chairs for spectators. Hotel conference rooms are an obvious one for smaller groups, though they may run you a pretty penny.
Y17 should have everything you need along with a great central location easy to get to by mass transport.
Tel +886-2-2343-2388
Address No. 17, Sec. 1, Ren’ai Rd., Zhongzheng District, Taipei City