To be precise, strumming when there is no note (strumming too soon or too late) will always produce a sound.
If you simply miss a note and don't strum at all, it should be silent.
If you miss several notes, you should not be able to hear the instrument at all.
If you CONTINUE to miss notes the screechy sounds for guitar and bass or clacky sound for drums will kick back in.
Apparently this keeps people from being able to jam along with their real instruments while the relevant instrument in the game is silent. Not sure why they would want to stop that from happening...