Out of the box, the Touch Board plays MP3 files stores on the SD card, you are not able to play these back at the same time, this can also be referred to as polyphonic playback, you are only able to trigger one MP3 file at once.
If you would like to playback sounds polyphonically there are a few options:
1. Set up the Touch Board to play sounds from the on-board MiDi library. http://www.bareconductive.com/make/on-board-midi-mode/
This will enable you to playback sound at the same time but you will not be able to play your own sounds. You can however select from 128 different instruments. Instructions in the Arduino code explain how to select the different instruments
2. Set up the Touch Board to play sounds stored on another device such as a computer or tablet, but setting it up as a MiDi interface device.
This will enable you to playback your own sounds at the same time but you will have to have your Touch Board connected to this other device at all times.
If you would like to have a stand alone option that play's your own sounds you could try our Pi Cap