Speaker doesn't play sound, but audio jack works. Fixable?

The headphone jack has a set of switches which directs the signal to the speakers when the headphone plug is not inserted.

A common issue is the contacts get messed up or are dirty. Using a good magnifier and light source you should be inspect the contacts and if needed clean them. Its also possible you have the tip of the plug stuck inside! That will take some efforts to get out.

My first guess would be the speakers are defective. Speakers are cheap, so even if the issue is something else, ordering unnecessary replacement speakers would not be the end of the world! I can't give you a recommendation on a replacement unless I'm 100% positive on your computers information, but if you enter your serial number into the site below it will give you all the information you could need. iFixit doesn't sell 2008 MacBook parts, but you can get them on eBay! Just search "*year* *model number* speakers".

Let me know if you have any questions!