Hi Phil,  The phenomenon you observed where a particular note
sounds loud in one listening position in the room and quiet in
another listening position a few feet away is one of the issues
that acousticians and audio engineers struggle with when designing
concert halls and other music performance spaces.  If you are
using a sound source (bottle) that radiates sound equally in all
directions around it, there are generally two major reasons that
could be causing a dip for some listening positions.

First, room "modes" are the natural resonances of the room itself.
The entire room will act like an organ pipe in sounding its own notes.
When a standing wave is set up at one of the natural frequencies of
the room, it will have nodes in the room where that frequency is
loud and it will have anti-nodes in the room where that frequency is
almost silent.  The natural frequency of the room will depend on the
dimensions of the room and their ratios to each other.

This is one of the reasons that recording studios are built with
non-parallel walls and with the room dimensions carefully chosen.
Unfortunately, room modes are at low frequencies where acoustic
treatments such as drape or fiberglass panels will be ineffective.
It is probably necessary to change the shape of the room if you want
to smooth out the modes.

http://en.wikipedia.org/wiki/Room_modes 

The second possibility is that a single reflection from a hard surface
is canceling the note at a given listening position.  For example, if
you have a hard wall behind your organ and the bottle is sitting away
from the wall a bit the reflection off the back wall can cancel a
particular note.

Imagine that the sound from the bottle travels directly to your ear and
also imagine that the sound from the bottle travels to the back wall
and then reflects back toward your ear.  Because it takes longer for
the sound to travel to the back wall and then to your ear, the phase of
that sine wave will be shifted compared to the one that went directly
to your ear.

For a few notes, that shift will cause the two sine waves to add to
nearly zero.  The notes that cancel will change as you move the organ
closer or further from the wall, but only in the example where it is
imagined that the back wall is causing the cancellation.

To find other surfaces that may be causing a cancellation stand where
the note disappears and have someone move a mirror along the wall
surfaces.  When you can see the organ pipe sounding the note, you have
found your reflection spot.  Since you describe you observe moving the
organ has no effect, it is probably more likely a room mode that you
are hearing.

A respected colleague wrote this accurate and thorough explanation of
rear wall cancellation:

http://www.genelec.com/learning-center/presentations-tutorials/placingloudspeakers/wallcancellation/ 

Even instrument timbres that are not as close to pure sine waves will
still have a dominant fundamental frequency that gets cancelled leaving
only the harmonics.

Which notes (with octave reference or frequency) are canceling?  If
you have any questions or want to discuss further please feel free to
contact me.

Best regards,
Dave Theis
San Francisco, Calif.