Sadly, with the design of the sound and RCMC boards, there doesn't seem any way to do uploads to the boards directly. Of course, that would also imply that Lionel would release the code to do such uploads, I think that's one sticking point.
Also I'm guessing, from looking at the RS-Lite board, there isn't enough RAM on the board to buffer the upload and then write to the FLASH. In addition, the serial link is only 3K BPS, so it would take a really long time to upload a significantly sized sound file. For a 1 MB file, it would take 55 minutes to load assuming you had the full 3K bandwidth available, in truth there is always quite a bit of overhead, so the time would be significantly greater. Next, the TMCC/Legacy interface is a uni-directional interface, so there is no possibility of knowing if the upload succeeded, you have have to do the entire upload and then see if the sound file passed the check on the other end.
Bottom line, I don't see uploading as practical given the design of the TMCC/Legacy operating environment.
However, if Lionel sees fit to give me the code, I could probably program them.