As this thread has largely petered out, I wanted to clarify one or two points because the information I have been trying to provide has been either deliberately or inadvertently misrepresented, either through lack of knowledge or some personal agenda.
For example, it's ridiculous to suggest that I am advocating filmmakers should not bother with sound or should give up. What I am advocating is for filmmakers on this forum to learn the difference between great, competent and poor sound, to be realistic about what is possible to achieve with a given budget and to use this knowledge to be a better, more competitive filmmaker, not to give up!
The simple fact is that cinema sound is the very top of the sound tree (the "Olympics" as someone put it). If you are an actual or aspiring professional, want to enter your short into a film festival (which takes place in a cinema) or screen your short or commercial in a cinema for any other reason, then like it or not, you are at the Olympics! As Wheatgrinder said, "Go to typical film festivals that would show your film.. Even the winners have dodgy sound.. ". This is my experience too. Very, very few micro-budget indie films ever achieve even decent sound (I'm not talking good or great sound, just competent sound). And, the fact remains that more films get rejected by film festivals for problems with the sound than for any other reason. This suggests that many indie filmmaker's concept of "decent sound" does not match up to audiences' or film festivals' concept of "decent sound".
From what I have written previously in this thread I would hope that the average indie filmmaker would take away the following information:
With no budget, the mix you create in your edit suite is going to sound dramatically different in a cinema and so there is no way to know how your dynamics or the balance between you audio elements is going to translate between your edit suite and the cinema. Nothing you can do about that, except to just do the best you can with what you've got. But, at least having an understanding of the existence of this translation problem may help you to avoid some of the pitfalls. In the case of the OP, if you've got music with lyrics playing at the same time as dialogue or some sound FX, it's extremely unlikely that you will be able to get the balance right (when played back in a cinema) unless you've got a serious budget for professional mixing. So if you want to use music with lyrics, try and place it where no other audio is occurring.
If you have a bit of a budget for sound, try and get the best, most experienced audio post guy you can. Never use a music engineer or producer unless you have no choice. Always remember though, that experience is only part of the equation. When hiring a sound designer/mixer ask for photos/information about their room. The further away their room is from the ideal of a top commercial mix room, the less likely it is that they can produce mix which will translate to a cinema. There's no exception to this rule, so don't allow yourself to be convinced otherwise! If you have a little more budget, see if it's possible in your area to hire a Dolby mix room for an hour or so, just to play your mix through. A good sound designer/mixer will be able to use the information gained from this test listening to adjust the mix back in his own room to reduce the most obvious problems and get it more "into the ball park".
One last point, just because it's difficult or near impossible for a micro or no budget film to achieve decent sound should not be taken as a reason to "give up" as MetalRenard implied I was suggesting. Even worse in my opinion, is to take MetalRenard's advice and to tell indie filmmakers their sound is good or decent (when it isn't), just to boost their confidence. Instead, the fact so few low/no budget films achieve decent sound should be taken as a great opportunity by the up and coming filmmaker. If you can be realistic about what's achievable with your budget, try to develop as much objectivity as possible about what constitutes decent sound and put as much into your sound as your budget and time allows, this will greatly help your works to stand out from the crowd of other low/no budget filmmakers. In other words, it could give you a significant advantage which should not be sniffed at in such a competitive field.
G