the ambient sounds used by Factorio, normally, are in data\core\sound\ambient. I'm not sure that any files there are used simply because they are there, or if the exe looks for specific ones, I'm going to assume the latter. Some other ways to play sounds are listed below:
Some entity prototypes allow you to specify a sound (typically for when they are 'working') by providing the sound path, you'd need to look at the wiki
prototype definitions (or I've tried to get some of the missing definitions recorded
here, zip in second post).
'Triggers' can be used to play sound, so you could use any entity's trigger property to play-sound (of course, you have to find which entity has a trigger property that you can manipulate, the simplest would probably be the generic entity's created_effect or the entity_with_health's attack_reaction). See information on triggers
here
I want to say that the MoLogic Core had a piece of sound code, but it was fairly limited from what I recall (you could ask it to play a sound that was part of MoLogic Core, but not your own).
I wrote a 'semi-useful' (at least theoretically) bit of code in an unreleased Extra Utilities mod/library that you can see
here (based initially off of the 'warning flare' from the old CC mod). See the files that have 'sound' in their name and the unit-tests.lua file, however this implementation will almost certainly be reworked to use the entity_with_health's attack_reaction trigger instead (I wrote this before I'd gotten the information about triggers gathered and on the wiki).
If none of those methods work for you, then there isn't a way (that I know of) to play custom sounds (and thus ambient music). In all cases, it would be difficult if not impossible to stop the sound once it was started (and for long sounds and some uses the sound might theoretically be played a second time before it was even finished).