When I heard the phrase: "Sorry to keep you waiting" [sɔri tə kip jʊ weɪdɪŋ] in an American movie it sounded to me that: Sorry, keep, and waiting are the stressed words. I may be wrong because I'm not a native speaker. I'm not sure about the degree of the stress. I'm curious when you pronounce the phrase above as a native speaker which words do you stress more. I know stress depends on the context, but let's say the context here is the most common way to say it, for example someone left a room and returns after a certain time. Any suggestion would be appreciated.
You are right.
However if you expand the sentence to "I am sorry to keep you waiting." then you can choose which of "sorry", "keep" and "waiting" you stress in order to modify the intended meaning. Do you, the speaker, want to stress how sorry you are, or that it's not the waiting but the prolonged nature of it that is bad, or that waiting itself is just something you are really sorry has happened?
You could in fact stress "you" to indicate that you might not be as sorry had it been someone else who had been kept waiting. You could stress "I" to show how deeply personally you mean the apology. You could even stress "am" to say how definite you are that you are sorry and that no one should doubt it. The only word that it doesn't make sense to stress is "to".
Of course you can't stress all the words at the same time and still make sense. I am not sure what the maximum number you can meaningfully stress at once in that sentence is but my guess would be three of them.