If filming a pub scene, there will be no sound played, as they would only need to hear the dialogue. So people the back would only be miming. But when it comes to editing the track they would add on the sounds(example below).
From the left it shows the sound is loud alot is going on there, as the sound wave is dark and long while on the right the sound wave is little and not as close together. It shows things are calm there and not as loud. When it comes to editing and dealing with it, the lines showing the waves make it easier to work with.
The first boxes represent a conversation. When editing a conversation it is best to overlap each section of it, so it would seem normal instead of sounding like block cuts.
During an conversation there tend to be music playing in the background (the bottom line in the diagram), to make the conversation more smoothing to listen to.
The sites that you may want to go on to download sound would probably be-
Wildtrack and Incidental music-both are pretty much the same thing as they are
both known as add on music/sound track