Leta is right. "giây" is "second(s)" and "dây" is "cord" or "string". Even though you had an audio exercise, we still must learn how to spell correctly.
If you were a student learning English, and your audio exercise was "They're going over there with their dog.", You're expected to learn how to spell "they're", "there", and "their" correctly. If you made mistakes, great! You can learn from them. Mistakes are valuable lessons. I hope that helps!
If I was giving you an audio only test in English, and I made you spell "Five pieces in a [w]hole", you could type "Five pieces in a whole" or "Five pieces in a hole", and I must accept either one, because both are valid concepts for those five pieces. Both are spelled correctly. Given either answer, it means you have already learned how to spell. Spelling is not the issue, as long as you answer with either "whole" or "hole". Without any more context than the audio context, this is called ambiguous. Ambiguous does not mean that we need to learn to spell. It means that there are more possibilities that are correct without more context.
So, again, given only the audio, weeliansoh is correct, and Leta is wrong. As weeliansoh suggests at the top of this thread, given only audio, it could be "5 meters in 1 second".
If the audio question is ambiguous, perhaps it would be simpler to change the audio question so that is does not require an ambiguous homophone in the answer.
I see what you mean now. Leta's comment about "giây" and "dây" being Vietnamese homophones is not wrong. We both misunderstood you and thought you're trying to claim that "giây" and "dây" are same words for "second". So we thought we could let you know.
"Five pieces in a [w]hole" is an excellent example about ambiguity, and I understand your comment about weeliansoh's suggestion. (Since we're here to learn Vietnamese, not English). I also agree that Duolingo should try to avoid sentences that have ambiguous homophones.
However, that is not the case with this particular Vietnamese sentence. "Five metres in a cord/string" could make sense in English, but "Năm mét trong một giây" can only mean "Five metres in a second" or "Five metres in one second". Even if we heard it. If one would like to translate "Five metres in a cord/string", then it would be "Năm mét trong một sợi dây".
Of course, one could argue that "Năm mét trong một dây" is valid in everyday speech and given the context. On the other hand, Duolingo is a language learning app, and their sentences are properly structured so that we can learn grammar properly.
Regardless, I hope that makes sense. Turns out you and I discovered that this exercise taught us a lot about Vietnamese vocabulary and grammar =)