To start play, click in the top line of the Trackwindow. If some bars on one track are selected, the events of the selection will be replaced by recorded events. If you start record/play with the right mouse button, the track will be muted before play starts and recorded events will be replaced. Klicking the left button, the track is not muted and recorded events are merged.
A click with middle mouse button (shift+left when configured for 2-button mouse) starts cycle record/play, the selected bars are played over and over again until you stop play by clicking into the top line again.
Also, you can start and stop play by hitting the space bar. When you start play with the mouse, jazz will remember the start position. On spacebar jazz starts play at this position again.