Voltage of sounds of MP3 files
Amplification

It does not depend on the music file, but at the amplifier sending the music to a speaker, or to a headphone/earplug connector.

Music file

A music file only contains 'values', and mostly in a compressed form. After uncompressing, you get values which has a certain amount on bits per value (typically 16 or more). 

Conversion

A microcontroller can send these values to a speaker via an pre- and/or amplifier, which converts it into actual voltages. So it depends on the amount of amplification.

Protection

Also note that sending continuous 'max values' do not result in a loud sound, actually it would  breaks the speaker (to prevent this, in the amplifier a so-called DC speaker protection is present). A wave is needed to let the speaker move outwards and inwards very fast, and this is done by sending changing values to the speaker. 

Measure

If you want to measure, you can use an oscilloscope to measure the audio output while sending a sine wave with a maximum amplitude.