Mini Tutorial....
1) Open Sonic Visualizer
2) import audio file.
3) Layer->Melodic Range Spectrogram ->All Channels Mixed
( you should now be able see the notes quite clearly )
4) Click the "Constrain playback to selection" and "loop playback buttons" (top toolbar just after the main play/stop etc buttons.
5) choose the "select tool" (towards the end of the top toolbar.
6) you should now be able to visually select a note by dragging over it and sonic visualiser will continually play that loop. If you want to listen to a new set of notes - just select them... playback will automatically detect the new loop.

you can use CTRL + page up and CTRL + page down to change the playback speed.

Hope that helps.