You can try the advanced editor, info below. However, AT&T voices do a very poor job responding to the advanced emphasis commands:
The help is a little confusing related to this. The phonemes button is only available in certain cases. IN particular, when using the AT&T Natural Voices via the SAPI5 interface. By default TextAloud uses these via SAPI4, because it is more stable and has a couple of features TextAloud uses that aren't available in SAPI5 (such as progress updates while speaking to an audio file).
Assuming you have version 1.459 or later of TextAloud, go to the Misc Options TAB and you should see a selection there for the sapi version. Set that to Sapi5 Only, then Options->Pronunciation Editor should have an advanced TAB available which will let you create new pronunciation for words using phonemes.