Wanting to power small audio amp toy with usb power bank
A fresh no-load voltage for alkaline cells can be as high as 1.63V (and higher at elevated temperatures), see data1 or data2, or up to 1.65 V according to Wikipedia. Therefore 3 of fresh alkaline cells in series can make about 4.95 V initially. The audio amplifier is likely tolerant to this kind of voltages, with some +15-20% room, so a +5V stable supply should be just fine. I see no danger to supply the device straight with +5V source.