Flash back to August 18, 2011. I was already looking through the Sonic Betas of the Genesis/Megadrive once again, and suddenly went, "Hey, I wonder if Project 2612 updated their Sonic 3 and Knuckles soundtrack?" I checked their site, and sure enough, they did. I downloaded it, and looked through the songs and immediately noticed one thing... the Sonic and Knuckles 0525 beta Credits Song is missing!
Fast forward to today. There are two of them on this particular ROM [and later ROMs up to the final Sonic 3 and Knuckles], though, that's probably what tripped them. The hex sound codes on that ROM are the usual 26 and the formerly unknown DC, located in the sound test. DC is the code you should play for that song. [Sonic 3 and the S3C beta ROMs simply play the splashing sound effect on sound code DC.] This track is the same as the final Sonic  and Knuckles Credits song, but the Sonic 3 Extended Ending song is tacked on after the Sky Sanctuary area instead of the final Sonic  and Knuckles Extended Ending Song. [around 2:30 on the recorded file linked below] The Sky Sanctuary Zone area in this track is also shorter than the final version song.
For the record, here is that particular track I am talking about: Credits v2 , and the final version for comparision: here. - I recorded both of them [amongst others] in WAVE format back in May of this year when I got my new laptop, revisited them later in August, and encoded them to MP3 format just today, 2011/09/11.
Although, if you already have the Sonic and Knuckles beta 0525 ROM, just play the hex sound code DC in the Sound Test. If you do not have the ROM, it is located here. Otherwise, the MP3 file is here.