Will my drywall shelf collapse if I put a speaker on it?

Any drywall wall built to US building code has a 2x4 or larger vertical stud every 16 inches or less. Finding the stud in a built-up and painted wall is usually accomplished by finding the attaching points, either visually if the wall was built sloppily, but usually by means of a < $20 purpose built metal detector called "A stud finder"

Once you find the studs, you attach the shelf or speaker to the studs. My speakers are on a long shelf about a foot below the ceiling along with various objet d'art. This shelf is supported via these brackets which are both WAF and structurally tested:

http://www.ikea.com/us/en/catalog/products/90136136/

Actually 16" centers are the norm, but 2' centers are to code if the walls are non bearing.
The shelf is called a bulkhead, not a wall.
Something to keep in mind is that drywall...1/2" and larger is considered self furring up to 24"......hence the 2' centers.
Bulkheads could be supported by studs as far apart as 48" if cantilevered from the studs on the wall ..... a bulkhead is considered a ceiling
if supported on one side.
Your bulkhead.....or cove if you like...may or may not take the weight.
Code has little to do with architectural features if theyre not structural.

I doubt that you are going to put enough weight on a wall to bring it down unless it is really flimsy. However, the chance of having the load rip itself off of the wall because you didn't anchor it well enough is pretty good. This would be the voice of experience talking, sadly. :-)

What are you trying to hang - 300 pound speakers or 15 pound speakers?

It shouldn't matter unless you are trying to hang your couch on the wall. ;-)

Generally the class of studs in a house or apartment are exposed a little some place. Got an attic? Go looking for them! How thick is the wall? The studs generally span the distance of the thickness. They probably aren't less than an inch thick. The nastiest thing about thin studs is actually getting your fastener driven into them, not its lack of strength to do a reasonable job.

What does the speaker weigh?

The last time I did this my first attempt wasn't too pretty. I redid it using the approach and hangers that I mentioned in my first post. That and running the speaker cables inside the wall made all the difference. My speakers were probably in the 20 pound range and they did require a little attention to detail.