This can be done with a pronunciation dictionary entry to automatically insert voice change tags in the text. This is only posssible with Sapi5 voices, TextAloud does not support voice change tags with Sapi4.
In this example, we'll use AT&T Crystal as the voice to switch to. Use Insert Tags -> Voice Change from the TextAloud main window to see what your voice change tag should look like.
Under the Tools menu click Pronunciation Dictionary Maintenance. Select a dictionary and add a dictionary entry. In the Add Dictionary Entry dialog, set up the regular expression and voice change tag like this:
<voice required="name = Crystal16">$1</voice>
The expression is looking for any one of the three characters [(", paired with one of the characters ])". Any text between these two characters will be spoken with Crystal. Note that the 'end tag' </voice> causes the voice to switch back from Crystal to the original voice.