[ the symbol for the volt is V, not v. It doesn't take a plural, so 5V, not 5vs ]
For LEDs you'd typically use a series resistor to limit the current to a safe value for the LED - normally
small LEDs are rated for 20mA or so.
For powering that MP3 player you'd probably need a voltage regulator to drop from 5V down to 3.7V.
(Which is an unusual voltage to drop down to, 3.3V is much more common) You might want to check
if the MP3 player specs say the minimum and maximum voltage it accepts.
Or use a 3.7V lithium battery to power the MP3 player and a boost DC-DC converter to generate
5V from that.