I was doing some reading after reading your post, and I was suprised at what I found. It seems that modern Clies only have Software Sound Synthesis, most modern Clies that is, but not yours - Sorry...
I really haven't been able to find out if something like the PEGA-SA500 would work, but it doesn't seem so. The article I was reading actually recommended getting an older Clie if you were interested in sound, as some of them have hardware sound support - my NX80V for one ( interestingly enough, it doesn't even support software sound synthesis - I think that they took it out to give it more ram room ) and it plays back MP3s excellently. Of course, even used the NX80V is alot more than the TJ27, but there are other older Clies that do an excellent job as well and don't cost as much - it really depends on what features you want. What I recommend is to sell your TJ27 ( I sold my Zire71 on eBay, and used the money - and what I got from selling my dedicated harddrive MP3 player on eBay - to buy the NX80V which I absolutely love. Take a look at this -