What's the difference between "nghe tiếng" and "nghe thấy"? I've seen then both translated as 'hear' in this lesson. Could I also write something like Tôi nghe tiếng cô ấy nói cảm ơn to mean 'I hear her say thanks'?
Also, the Tips & Notes say that "thấy" can be used to mean 'already', so could you translate Tôi nghe thấy cô ấy nói cảm ơnas 'I already heard her say thanks'?
"nghe tiếng" is hearing someone/something (when they made a noise), while "nghe thấy" is hearing someone/something saying/singing something. So one is when you hear without being able to discern the context of the noise, the other is when you can hear and understand the noise.