Good question - there are some recordings on youtube as a result of different finds with different detectors. For example - a 5000 finding gold etc etc. These may be a start. Go to youtube and check against the detector you are using. In Victoria the sounds will differ from a beach or park detector sound due to the mineralised soils. For example, a VLF will be quiet in a park, but make a hell of a racket on a Vic goldfield. Again, EMI will make the sounds different, hot ground (red clay domes and the like), hot rocks, pipeclay, charcoal etc etc. Lancair is spot-on - you have to basically build up a mental 'sound memory' over a period of time for each different area you are in and the ground can change rapidly from area to area in the space of a few metres some-times. If starting - get a small test piece of lead (or gold) and detect for the test piece on different grounds and at different depths - pretty soon you will learn - a. the machines capabilities and b. the different sounds you will get.