The MIDI track is getting level from the MIDI controller, but the Stereo Aux. Input track is not getting any signal when using SampleTank. This is assuming that Pro Tools, OMS, and SampleTank have all been setup correctly.
Try pressing the play button, if the error -9135 appears, this is the reason the Stereo Aux. Input track is not getting signal. A -9135 error message means that Pro Tools LE is not communicating with Mbox.
Troubleshooting this issue should now focus on solving the -9135 error message. Then, the Stereo Aux. Input track should get a signal level.
First of all, the Mbox MUST be connected to a USB port on the computer, not a USB hub. If that is the case follow the troubleshooting tips below.
Steps for troubleshooting the -9135 error:
1) Trash preferences. Mac Hard Drive > System Folder > Preferences folder. Drag to the trash: Digisetup and Pro Tools LE prefs. Empty trash. Shut down.
2) Plug the USB cable going to the Mbox to a different USB port on the computer. Restart.
3) Launch Pro Tools LE and try to playback.