I actually got pretty good at telling coins apart by the sound of the single tone.... More by the length of the tone (width of target probably), than a change in tone..
Practice, practice, practice... But with the single tones, bottle caps will always fool you... A trick there is if it jumps between penny/dime, and quarter, move the coil just to the outside of the target until the tone fades, then slowly come back, and if it is a bottlecap, it will read "iron".. That tip has saved me many wasted digs...