iMac has a usual integrated sound card.
However: while professional sound cards do provide substantially less noise than the integrated sound cards, both are well below human ability to distinguish sound. In short: it does not matter whether you have an external or integrated sound card you WILL NOT hear any difference, because you ears simply can't perceive it. External sound cards make sense only if you want to record music onto media or need control over the output.
By the way, most people who claim to hear "better" sound with "dedicated" sound cards just forget that these card often have a built-in equalizer. Of course it will sound differently from the integrated card, but in no way "better" if we define "better" as the ability to reproduce the original signal more precisely.
So, don't worry, iMac's integrated sound card will be sufficient for any audiophile (or not) speakers you can throw on it.