At times, I also have trouble hearing audio from my MacBook. I've done lots of searching and have never found a software solution. The only thing that I do to resolve this issue is use earphones or external speakers. Another option is using
VLC player to play media files. I have found that VLC has a less restricting peak volume. Finally, I'm sure you have already considered this but just in case, be sure that the application you are using to play media has its own volume on max. iTunes, Quicktime, VLC, Real, etc all have their own volume control independent from the system volume. Also in iTunes, its possible to increase volume for a single track or song in its info.