This is pretty... frustrating to say the least. How come that some people (e.g. Phil) report that it works in the first place, then only with modifications, then not at all...
Anyway, thanks for the warning, but apparently I had to learn it the hard way and already populated it myself a few minutes before you wrote your post 😀
Although the cap values were slightly off, I had the same result as you. Even with the caps, no improvement whatsoever. I tried a few sound cards, SB Audigy, ES1370, same thing (unfortunately, I don't own a fancy YMF7x4 card).
I already knew the post about the -12V rail, but I dismissed it, since it was not about the T5720. Stupid me.
Concerning the buck converter, they are only available from China and I am not willing to wait 2+ months. So, I ordered the only one available short-term and it is absolutely humongous, 4x4 cm! omg, this will be the ugliest thing that ever was.
At least, you can grab +12 V pretty easily right next to cn4, but the output will have to be soldered directly to the socket, since there is no connection at all between B1 on the socket and the riser male.
I hope, you will post, how it went for you, when you get yours.