Hi,  When I built the Wolfe Calliope about 30 years ago, we had the
same problem.  In a parade, it was okay because it was just passing by
viewers on the street.  But when it was stationary in a festival or a
show, it quickly bothered the people nearby.  So, I built a muffler for
the pipes.

In order to keep it as short and small as possible, I didn't enclose the
outer row of pipes, which were are the longest.  It turns out they are
the least objectionable, too, since they are also the lowest frequency.

So I measured a rectangle that would fit neatly between the outer row
of pipes and the next row inside.  And, I measured the height of the
longest pipe that would be inside the finished box.  Then I made a
rectangular, wooden box from 1/2" plywood that was about an inch higher
inside than that longest pipe (a top, two sides, a front, and a back).
It was put together with finishing nails and glue.

If you don't have some place for the air to escape, the box will bounce
up and down as you play the calliope.  I made a series of 1" diameter
holes (about 1/2" apart) lengthwise down the center of the top of the
box.  The row of holes was centered left-to-right and a total length of
about a foot.

Then I cut a piece of plywood that was about 7" by 18" and screwed it
over the holes, centered in both directions.  I put a 1/4" spacer
between the top of the box and the plywood 'cover' to let out the air.
The 1/4" space will let out the air and (because it is narrow) it will
act as a frequency filter to reduce the high frequency sound from the
pipes.

I painted the box to match the calliope (red and gold), added some
trim, and put weather stripping foam on the bottom edge.  I added a
couple "U" shaped kitchen drawer handles on the top to make it easier
to handle.

It worked like a charm.  We even played the calliope _inside_ a shopping
mall with no complaints from the nearby stores.  We did only play a few
tunes at a time, not constantly.  Good luck!

Regards,
Craig Smith
Upstate New York