From Joanie. Found whilst testing the new application specific 
settings feature (see bug #363815):

My default voices setting is:

orca.settings.voices = {
'default' : orca.acss.ACSS({'average-pitch': 5.0,
 'family': {'locale': 'en_US', 'gender': None, 'name': 'Paul'},
 'gain': 9,
 'rate': 65.0}),
'uppercase' : orca.acss.ACSS({'average-pitch': 6}),
'hyperlink' : orca.acss.ACSS({'average-pitch': 2}),
}

I decided to make Betty my default voice for gaim:

orca.settings.voices = {
'default' : orca.acss.ACSS({'average-pitch': 5.0,
 'family': {'locale': 'en_US', 'gender': None, 'name': 'Betty'},
 'gain': 9,
 'rate': 65.0}),
'uppercase' : orca.acss.ACSS({'average-pitch': 6}),
'hyperlink' : orca.acss.ACSS({'average-pitch': 2}),
}

Sometimes Betty kicks in; sometimes she doesn't:

1. Launch Gaim, arrow around in the buddy list, navigate in the menus:  Paul

2. Alt Tab to get to some other window, but don't release Alt (i.e. just cycle
through them):  Betty

3. Type something in a channel with key echo enabled (or review your text with
the arrows prior to pressing enter):  Paul

4. Press Enter to send the text to the room, causing Orca to speak that text: 
Betty