I've said this a few times over the years to both novices and experienced prospectors alike.
Do NOT listen for a particular signal/sound. Those that say they can tell the difference between junk and gold are kidding themselves.
Listen for a repeatable change in the threshold.
It can be tonal change, level change, drop outs, warbles, screams, faint murmurs, whispers.... whatever...
If its repeatable, dig it.
If you aren't sure, take a few cm off top and check.
If you're still not sure, dig it.
I have pulled gold at shallow depths that screamed like junk. I've also pulled junk that had the sweetest deep mellow tone you'd normally associate with a nice nugget.
The range of shapes, depths, density and purity of gold nuggets doesn't allow us to establish baseline sounds.
Moral of the story.... DIG, DIG, DIG