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..

via What is SID ? –                    The Destroyer’s TI99/4A Page.