The pneumatic compressor (I believe it's [partlink:x191c01]) can be used with a motor to produce a continuous stream of air for as long as your batteries last.

You'd still need to find a way to either move the hose or direct air through different paths - as was shown in the [points question][1], linear actuators could be used to move the hoses with precision, however they may not be fast enough to play the harmonica:

![Small Actuator][2]

However, if you are looking for a more manual process you could use [partlink:4694b] Pneumatic Switch which allows you to direct air flow through one or other of the two holes, with air being provided to the lower, central input. 

  [1]: http://bricks.stackexchange.com/q/512/56
  [2]: https://i.sstatic.net/9OsOl.jpg