There is going to be a screening of the film soon, and I have been told that, in order to sound well in the theater, the level of the music must be in the range of -16dB to -6dB.
I never had to deal with such an instruction in the past. I usually simply make sure that the level meter doesn't go in the "red zone"... Is there a way I should proceed to make sure that I respect the range?
As MetalRenard said, the level of the music is irrelevant, the only relevant level is the level of the entire sound mix. Your music could be playing in the background, say at -18dB for example, so would be too quiet for the specifications given. At the same time, with some shouting, explosions or other sounds in the sound mix, the levels could be at say -0.1dB, which would be too loud for the specifications given.
Even if they are talking about the sound (rather than just the music) mix being -16dB to -6dB, this is a completely bizarre specification for a theatre, in fact it's a completely bizarre specification for anything audio; TV, DVD, internet or even a commercial music CD. This spec is way too loud for a TV mix and a TV mix is already way louder than a theatrical mix. Played back in a standard theatre, a mix at these levels is going to have the entire audience immediately run out of the cinema.
The problem facing film festivals where the screening takes place in a cinema is that indie filmmakers do not have the equipment to create a theatrical sound mix and usually have no idea what they are doing when it comes to mixing sound. Some submissions are at levels only appropriate for music CDs, others at levels more appropriate for TV and some attempt to achieve theatrical type levels. The difference between these levels is absolutely immense, very roughly speaking a mix to music CD standards is likely to be 20-40 times higher than a theatrical mix. In other words, a sound mix submitted at music CD levels would likely have to be turned down by roughly 30dB for playback in a cinema otherwise the audience would run screaming out of the cinema with their hands over their ears.
So what festivals have to do is test every film to be screened, make note of an appropriate playback level for each film and then change the playback level before each film is screened to the audience. Obviously this takes considerable time, so it seems as if this particular festival is maybe trying to avoid this whole testing scenario by specifying a very small range of extremely high dB levels (-16dB to -6dB). In theory, they can turn down the cinema playback system by say -20dB or so just once (before any of the screenings start) and then just reset the levels to the normal theatrical setting once all the festival screenings are finished. This makes it easier and cheaper for the festival but means there is no opportunity for the filmmakers to use any more than a tiny fraction of the dynamic range of which a theatrical sound system is capable. To give you some idea, TV usually has a dynamic range of 20dB or so and film can be 60dB (these are extremely rough figures btw). In other words, a theatrical system can use roughly 300 times more dynamic range than the 10dB range you are being allowed!
BTW, making sure the level meter does not go into the "red zone" is fine for music CDs but would be unusable for TV broadcast or for theatrical sound.
G