Yes, the turtle beach santa cruz was one of them.

Based on the cs5630-cm crystal chipset.
I'd imagine, some of the hercules cards could possibly do that as well (similar chipset)

I don't know how the santa cruz does hardware mp3 mind. I've yet to work it out on mine.

All the Crystal soundfusion based cards (CS46xx) can do mp3 decoding. There is a DirectShow filter driver cwcmp3.ax that needs to be installed somehow, and a microcode file cwcmp3.osp. Then you can use DirectShow compatible media players like Mediaplayer Classic for hw mp3 decoding.
But I wouldn't bother with this too much, the decoder is partially broken, sound will get garbled after ~1min of playback or so.

Is it not installed by default then?

I think it's not even included in he santa cruz driver package (maybe because it's broken...). You have to grab a generic driver package for the cs46xx chips and extract the files from there.

Meanwhile, I've found this old thread - MP3 hardware decoding ISA cards

I think the Sigma Designs Xcard has mp3 decoding in hardware.