Found the clip on twinpeaks.org although I guess I got the scanned letter for MD mixed up with the usenet post. Here it is:
To: Theatre Projectionists
Re: Twin Peaks: Fire Walk With Me
David Lynch, the director of Twin Peaks: Fire Walk With Me
has asked me to contact you regarding the sound level of
his motion picture. Mr. Lynch has put a lot of effort
into the soundtrack of Twin Peaks, and feels that the best
reproduction of sound will be achieved by increasing the
volume 2 decibels above normal. Your efforts to
accommodate Mr. Lynch will surely result in greater
audience enjoyment of the film and, therefore, greater
box office sales.
Thank you.
http://www.twinpeaks.org/faqfwwm.htm#f7
-B