If your guitar puts out a line-level signal or you're using a powered mic, fine. So, the MBP is expecting a line-level signal, meaning preamplified audio. No 'extra' port that way, and USB headphones are cheaper than an iMic.Д®DIT: The caveat there is that the audio in on the MBP (true for all MBPs, not just the 13') is a line-level input, not a microphone input. That would be fine as well - set the MBP's audio port to be used for input, and connect the USB headphones and use those for output.

Looking back at your first post, you mention USB headphones. Thinking on your comment, I see what you mean by 'an extra port' - the iMic has both in and out, meaning you'd actually have three counting the internal port on the MBP. It mentions iPhone headphones, but those will work only as headphones, not as a combined mic/headset when connected to the MBP. This means you must choose whether you want the single audio port on the 13' MBP to be input or output. You can change the order of these devices. Next, look at the devices listed under вЂSubdevicesвЂ™. Next, open the Clock Source dropdown and select one of the audio output devices. Check the box next to each device you want to output sound to. Specify whether you want the port to be used for audio input or audio output. Specify whether you want the port to be used for audio input Select it and the pane on the right will list all the audio devices that are connected to your system. You can specify audio analog line in functionality, to support line-level microphones and audio equipment, by selecting Audio In in the Sound pane of System Preferences.'+ +'Choose Apple ( ) > System Preferences, and open Sound preferences. +'Choose Apple (пЈї) > System Preferences, and open Sound preferences. You can specify audio analog line in functionality, to support line-level microphones and audio equipment, by selecting Audio In in the Sound pane of System Preferences.'+ +'*Audio out port.* Connect external speakers, headphones (including iPhone), or digital audio equipment.

Mac pro audio input and output