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They say "счастливы", with the stress on the first syllable.
"Счастливый" (the stressed syllable is bold) sounds different.
What might confuse you is that there are two words счастливо which are...

No, they are speaking Russian, and they are saying будьте счастливы, and that's the correct standard Russian pronunciation. I don't know why you think you hear "счастливо". Actually "счастливо", with...