The level issue is a real one depending on the device you're using, and beyond that you often encounter mp3 players which have sufficient level but the bass output is utter crap when fed through a band PA.
If you need to go that route, a more flexible option is to have a cable that takes a microjack/headphone output from an mp3 player and splits that into a pair of 1/4" jacks.
You can then plug those 1/4" jacks into the L/R channels of a stereo strip on the PA, giving you access to far more control of level PLUS access to the EQ section.
These cables are cheap - there's no real excuse for not having a set of red/white phonos and a set of 1/4" jack types with you.