iano, you rock.

from the site:

Sid Master 99 is a PEB interface cards that integrates an MOS 6581 or MOS 8580 SID chip into the architecture of the TI99/4A as a memory mapped device.   It can be accessed through any  language that allows normal CPU memory access such as assembly language, Extended BASIC etc.  If you can do a CALL LOAD or equivalent then you can access the SID chip.   A new vista of never before heard sounds can now be played through your TI99/4A.  No sound cables required.  It is completely compatible with the existing sound chip on the computer and plays through your monitor or TV...

http://www.dsapsc.com/what-is-sid.html