How is the Selenitic Age solvable in Myst?

If i remember correctly, the first intersection you come to the game will play a sound. At this point, there is only one way to go -- so that direction is that sound. Continue on, and it should be pretty straight-forward (maybe one or two backtracks).

I didn't even know there was a key to the sounds anywhere. Just deduction.

If you track your way through the Selenitic Age, you will begin to see patterns in the paths you take and that the tracks will eventually "loop" around (probably just defaulting back to the start). You can determine the full, correct path via this method although I do not recommend doing this at all.

There are no loops in the Selenitic Age maze. Consequently, you can map it out using either the right-hand rule or the left-hand rule for maze-solving to reach the exit (the right-hand rule, ie. taking the rightward-most tunnel every time you reach a junction, is faster than the left-hand rule).

A bit late reply, but:

In the book about Selenitic Age in the Myst library, Atrus suggests that the mysteries of the selenitic age might find an answer in other ages. This is a suggestion for the player to do this age as the last one.