The right way is
Los auriculares Because it is the plural form of the word, the singular form would be
auriculares is a valid word, but for me (I'm from Mexico) it feels more natural to say
Something important to clarify is that seeing how
auriculares is a plural word, and since it could cause confusion, think of it as something similar to
pantalones (there's only one item, but it's the right word to use). I guess the easiest way to understand it is that each side of the item is an
auricular, so we could say you have an
auricular on each ear when you use your headset, and you're using/wearing
auriculares as a whole.
For the sake of improving your Spanish, I have a few tips...
First, the right overall expression would be:
Los auriculares (or audifonos if you want) fueron caros.
Note the right word for expensive here is
caros (plural form) and not
caro (singular form) for the reason I explained before.
Finally your questions are:
¿Cuando hablando sobre un par de auriculares en español, cuál es correcto?
which is the wrong way to ask. You could do it in the following ways instead:
¿Cuando hablamos sobre un par de auriculares en español, cuál es la expresion correcta?
Hablando sobre un par de auriculares en español, ¿Qué es lo correcto?
And your other question:
¿Y también, es auriculares el palabra correcto por esa cosa?
That one is also incorrect,
palabra is a feminine word, so instead of saying
el palabra you have to say
la palabra; the same is true for
correcto which should be
por should be
para. So your question should be:
¿Y también, es auriculares la palabra correcta para esa cosa?
I'd personally use
esta instead of
esa as you were talking about that item just before...