"can I run an mp3 in the background and play along with it through the studio one interface"
Assuming you are using Studio One "Artist", mp3s are not supported. You would want to either use a wav file instead, convert your mp3s to wav, purchase the $10 mp3 add-on, or upgrade to Professional.
The add-on can be found here - https://shop.presonus.com/products/studio-one-add-ons/MP3-Converter-for-Studio-One-Artist
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"Can I use 1 set of headphones for both the pc and the interface"
Sure. You would need to connect the headphones to your interface then you would want to follow the guides at the links below:
1. Set the interface as your audio device -- http://support.presonus.com/entries/21486905-How-do-I-set-my-AudioBox-as-my-computer-s-default-output-
2. You might also want to disable "exclusive mode" -- http://support.presonus.com/entries/23177644-How-do-I-disable-exclusive-mode-for-a-Windows-audio-device-
