I had the same problem with my homemade E-caller from a year ago. With the tape player it sounded good but with the MP3 there was very little volume.
With my Sony walkman tape player i didn't need the PRE-AMP only the Audio Amp. With my model(sonic) MP3 i needed BOTH pre-amp and audio(out) amp.
The problem is with the low quality of the RS amp it tends to amplify "noise" on the pre-amp side of things and hence you get a lot of static. I am currently trying some different amp that are of better quality and I'm also building my own amp but don't have it completed yet. If i find anything of value i'll post it.
BTW, i have a FOXPRO and love it but building my own E-caller and trying to make it work is fun. I'm using a 7 Watt horn speaker, MP3 and Walkmans, and Nady wireless setup. I can also just use the player, amp, speaker and wire as well.
