Learn French: Le son de cette voyelle est très fluide, n'est-ce pas ? - The sound of this vowel is very fluid, isn't it?

In French, n'est-ce pas is a fixed, universal tag question. Unlike English, where the tag changes to match the subject and verb (e.g., "isn't it", "don't they", "hasn't she"), French uses n'est-ce pas at the end of almost any statement to ask for confirmation.

In this sentence, does fluide describe le son (masculine) or cette voyelle (feminine)?

It describes le son (the sound), so it is in the masculine form here. However, because the adjective fluide ends in an unaccented -e in its base dictionary form, it doesn't change spelling for the feminine. It would be spelled fluide either way.

How can I tell that son means "sound" here, rather than the possessive adjective "his/her/its"?

The definite article le gives it away. Possessive adjectives like son, sa, and ses replace the article, so you will never see le son to mean "the his" or "the its". Because the article is present, son must be the noun meaning "sound".

Why is it cette voyelle and not ce voyelle?