See also the resources page.
Based on reliable opinion:
Your best bet is to find a vendor of secondhand or refurbished SGI DAT drives, such as the following notified to me by others. (Disclaimer: I have no business relationship with any of these companies; in fact, so far I've had no business with them. This is not warranteed, and they may be out of stock. Be polite.)
For general advice on firmware versions and how to obtain it, consult the Computall DAT FAQ.
Note: As a general policy, I won't host firmware and related software here unless I know for sure that it is freely, legally distributable. Please email me if you are unable to locate particular items and I will do what I can to help.
N.B. Neither myself nor anyone else takes responsibility for the firmware available via this page and what it may or may not do. If you choose to install it, you live with the consequences. If it breaks, you get to keep the pieces. ;-)
If you have problems obtaining firmware or upgrading it, please email me, as I may be able to help further.
Someone asked how to upgrade a DDS drive using the firmware above. I haven't tried it, lacking a suitable test case and time. You need an OEM firmware upgrade program (usually called FUP, see above for a link to Dell's), commonly available from their support FTP sites as disk images or patches. Do not use OEM firmware though! From there, presumably you can copy the appropriate files and use the utility to install the audio firmware. If you succeed, let me know how you did it. (To people who keep asking me how to use these files: this is all I know!)
Note: recent reports suggest the firmware above can only be used by blowing it to a 2Mb EPROM and fitting it into a compatible drive. I.e. None of the upgraders will work if the EPROM is One Time Programmable (OTP) rather than flash. If you have any advice or pointers to advice on burning EPROMs, please let me know (see also emails).
Quit fannying around and put audio capability in your drives once and for all!!