Midi playback problem!

[Liam8mail]

Hi,
I'm using protools express with an mbox and a m-audio keystation midi usb controller. At first the controller would stop working randomly. I tried the usb powersave and so on. Eventually swapping usb hubs seemed to work. Now the problem i'm facing is at some point while working on a session the midi stopped triggering audio.

The controller is picked up by protools, the midi notes are recorded on the instrument track but there is no sound. The Midi works fine on other sessions. So I'm stumped as to why I stopped working on this one... MIDI through is checked. I played around with changing the input slot for the VI a to b to c. the output is to earphones. I put on input to check after reading it may help but no use.