We need at LOT more information.
like, what software and equipment are you using?
Give us everything you can.

Just to clarify and in case you didn't know, though I'm sure you do, midi does not contain audio only data. So you need to have something generating the audio like a virtual synth or hardware synth module. From there it's just a matter of signal flow. So long as your audio source is routed to your output, you should be able to hear sound. If not then the signal flow is broken somwhere. What gear are you using? What software?

thanks a lot guys!
i'm using protool 7.1.1, macbook pro, mbox2, triton le. i did the same kind of recording before, and the audio tracks worked fine. for the current project, everything seems going fine by the stage of recording from midi to audio, but got stuck since no audio sound can be heard.
i checked the set up connections carefully adn cannot see anyhting wrong.this could be caused by some basic thing which i don't know. i'm a classically trained muscian and just started exploring the recording world to produce my own music. i will appreciate your time and thoughts. thanks!

CX, just taking a wild shot at this, it sounds like perhaps you haven't assigned the proper MIDI tones/sounds/voices to your track, if you have, something's muted.

In any case, your time would be much better spent posting on the Musicians forum (a little further down the main page) and specifically in the "Sound and Rythmn" Forum. MIDI is discussed there specifically, along with a lot of other similar stuff (Sequencers, sound modules, etc.)

Good luck with it all!

One other odd possibility. Where are you monitoring from? Your monitors should be plugged into the Mbox. You likely won't hear anything from the Macbook pro's output. And, of course, you do have the audio output from the Triton going into the Mbox inputs right? Sorry I know I'm swinging wide here. Just trying to open up the possibilities.