On the MyCommodore64 blog you can read all kinds of information about sound cartridges. You can find specifications, look at pictures or listen to sound samples. A few examples of cartridges are: Cynthcart 1.2.4, Drum Studio, Magic Voice, Easy Speech 64, VoiceBox, CommTalk Speech Unit, Voice Messenger, Currah Speech 64, Votalker and ProVoice.