I have added voice feedback to my application for turn-by turn feedback. I pause the audio or radio for the time during which the feedback is given and then resume the music back . I am using Microsoft SAPI with Microsft Mary's voice and now planning to use AT&T natural voice but its very expensive. Take a look at the demos at
http://www.naturalvoices.att.com/demos/.
I can share to code for TTS in case any one needs it.
Regards
Satish