This is a hard question and depends on your target audience and/or wether you have the time to do several renderpaths. I can only talk for my own projects, but the user's base graphics cards range from very old and integrated ones to the latest ones around the corner so I try to support most of them in "Phase 2" of Projekt W via multple render paths. One for very old cards that don't even have shaders (using the fixed function pipeline, e.g. texture combiners etc.) and one with all eye-candy enabled that uses OpenGL 2.x functionality.

But you should at least settle for OpenGL 1.5. I don't think that there are many GPUs out there that won't support that one, and if you plan to release in one or two years you can even base anything on OpenGL 2.x.

As for 3.x and 4.x. Yes, in theory these are nice cause the forward only compatible contexts really get rid of a lot of old-fashioned and deprecated functionality. Those are the clear future of OpenGL but if you'd only use these you'd have a pretty small and limited user base as you need pretty new cards for those OpenGL versions.

So if you plan to roll out a game and want to reach as many people as possible you either have to go for the lowest supported version for your target audience or you implement several render paths. But you can make life a bit easier if you e.g. don't use immediate functions (glBegin, glTranslate, glRotate, etc.) anywhere, not even in your OpenGL 1.x render path but use vertex arrays or VBOs there cause these can still be used with the new OpenGL versions.

And as for OpenAL : It's API is based on OpenGL's API, but that's about it. OpenAL is still proprietary as it's owned by Creative Labs. So the only thing that both have in common is a similar looking API.