A decent silence is very hard to achieve in a music lesson.
One temptation is to overpower the fidgeting and background chatter by winding up the volume – either of your voice, or of the sound system. This will work, but is very tiring for ears and voice, and there is always the risk that everyone will just chatter more loudly!
A more effective method can be to reduce the volume or the sound system, or your voice, so that the class has to listen intently to catch sounds.
I often run listening activities using just the speaker on my mp3 player. A double class of 50 children crammed into a single classroom can be extraordinarily quiet if they feel it is worth their while. This only works if they can actually hear the track, or your voice – I came a slight cropper the other day playing “Peter and the Wolf” to a class on my mp3 player (because the sound system wasn’t working) only to find that the recording levels were really too low. Even so, the children, normally a fairly restive bunch, concentrated hard and managed to get most of the section of the story.