The Sound of Music
Music can make a film in the way that it helps suspend disbelief, maintain emotion, and help drive the narrative. Documentary film is even more reliant on it in some ways because it is one of the few elements of artifice that help elevate the material from a record of events to a film of artistic merit. Selecting the music for this type of film is incredibly difficult and requires knowing your purpose and acknowledging your limitations.
The truth is that unless it is one of the most experimental documentary films ever produced the soundtrack is going to be composed mostly of talking, whether it is from direct cinema footage or sit down interviews. What this means is that spoken word from any other source will likely distract from the content of the conversation. Keep this in mind and try to avoid using music with words when people are talking because it can become jumbled. This does happen at times, but try and keep it to a minimum and if you have to do it keep it somewhat quiet. If you are doing a montage sequence lyrics are fine, even preferable to add emotion and to maintain the audience's interest.
The appropriate mood is important for the music, but so too is style and genre. Generic music is an easy way to communicate with the audience quickly because they will usually associate it with much of the archetypal history of that genre, even if they do not know a lot about it consciously. This is a form of symbolic representation, where you use a certain branch of music to associate the scene within a form of precedence or historical context. For example, if you are talking about the creation of a radical commune in the early sixties, having some psychedelic folk music in the background will immediately give it context.
You do not want to rely on style completely because this just reinforces classic clichés and does not attempt to create a completely original emotional moment. Watch the scenes over and over again and see exactly how you feel about it. Then look into the types of music that you associate on a very personal level and then try to apply those. This is a way of adding absolute personal creativity to the film, and is the opposite of bringing subjectivity.
You may find that looking for songs that aid in transitions is a great tool. If a song has a lyric or sound that is perfect for bringing together two clips then you are going to see that your work as an editor is suddenly much easier.
Always be honest with your music and make sure that it fits the mood, style, and context of each clip. Do not let it compete with your footage because that must be the most important. Remember, more is not always going to be better when it comes to music in your documentary.