Paul Hesketh used a Teensy 3.2 to make a physical MIDI controller that controls a software audio plugin called Kush Hammer EQ.
A VST plugin is a virtual synthesis add-on format used by Digital Audio Workstation (DAW) software on PCs. DAWs such as Ableton Live, Logic Pro or Audacity are used to edit audio files for music, podcast and so on. VSTs have lots of complex controls and Hesketh thought it would be much more fun to adjust sounds with a physical controller rather than repetitively pointing and clicking with a mouse.
Hesketh designed his hardware to work with a VST plugin called Kush Hammer EQ, which is based around two channels with three frequency ranges each. As well as the Teensy 3.2, Hesketh used an EEPROM module, a PS2 keyboard driver module, an LCD display and an assortment of buttons, switches, dials and rotary encoders to complete his project. He even got a custom metal enclosure made, which adds to the sleekness of the finished controller.
If you want to have a go at making your own version of this project, Hesketh has released a detailed write up (PDF) of this project including his design process, build instructions, components used and an excellent breakdown of his code.