I've probably built over a dozen or so E Callers over the years.
I have used Cassette Players, CD Players, MP3 Players and the little Cass Creek Callers as the sound source. The best speaker I have found is the Speco SPC-5 (something about the frequency responce of this speaker, and such things I don't understand).
For Amps I have used the Radio Shack Mini Speaker Amp, generally removing it and mounting the Amp Cirtuit Board and Battery in a Radio Shack Project Enclosure or a large Coffee Mug (on the Coffee Mug Callers the Speaker was mounted in the Coffee Mug also).
The one I am currently using has a Cana Kit 12 VDC Amp that I have housed in a Radio Shack Project Enclosure, along with a SLA 12 Volt Battery. For a sound source I am using a RIO 600 MP3 Player (it was cheap off of e bay). I 21 or 22 tracks on it, some are Western Rivers downloads and others from Johnny Stewart Calling CD's that we ran through Gold Wave Sound Editing Software to isolate one sound loop so as to not use up so much hard drive space.
Only your imagination will limit what and how you can build one of these callers. Some guys have added NADA or ADZEN WLX-PRO Wireless Remotes for Video Camcorders to make these callers remote. This is a plug and play type of arrangement, plugging the amp & speaker into the receiver and the MP3 Player (or other sound source) into the transmitter with 1/8" mini plugs.
Larry