When you open a project from the original Soundtrack application in Soundtrack Pro, stereo tracks that include (or "make use of") panning effects may sound incorrect. They may sound like they are missing some or all signal from the channel opposite the pan direction when opened in Soundtrack Pro.
This happens because Soundtrack Pro handles stereo track panning differently than Soundtrack does. The pan slider in a Soundtrack Pro stereo track acts as a balance control (sometimes referred to as an isolating panner). With this type of pan, when you, say, move the slider to the left, the right channel is progressively removed from the signal.
Soundtrack features a collapsing panner in its stereo tracks. With this type of pan, when you, say, move the pan slider to the left, both channels of the stereo signal are progressively moved (collapsed) toward the left speaker.
Projects that you saved in Soundtrack that feature stereo track panning will not exhibit collapsed panning behavior when you open it in Soundtrack Pro. Instead, the panned signal may be faint or inaudible.