Headphone Plug

You have the model 1 SMS?

If so, it would be easiest to simply make an adaptor for the A/V output so the video goes to an RCA plug and the audio goes to a standard 3.5mm headphone jack. You will only get mono audio from the SMS.
Ah well, then you need to solder to the correct points inside your SMS. Using Mike G's RGB guide (it also tells you about the audio connections) solder the audio and ground connections to a standard headphone jack. Mount on your SMS somewhere and presto!Headphone jack created. Note that the stuff about mono still applies here - you will have to bridge the two tips of the headphone jack to make dual mono if you want sound in both ears.

Also, you will have no control of the volume, although the line signal supplied by the SMS should be enought to drive a set of headphones at an adequate volume.