Speaker as in someone talking. Or a Timing Prompter or a Seconding Prompter.

[link|http://www.dsan.com/SpeakTimers/SpeakTimers.asp|http://www.dsan.com/...s/SpeakTimers.asp]

[link|http://www.talklight.com/index-2.html|http://www.talklight.com/index-2.html]

[link|http://www.leonaudio.com.au/conf_tmr.htm|http://www.leonaudio.com.au/conf_tmr.htm] in Austrailia

[link|http://www.technicaldirections.net/speakertouch.htm|http://www.technical.../speakertouch.htm]

Prices vary, but some of these are spot on what you want.