I am just starting to learn Thai, and I can't figure out why the word  น้ำ is pronounced /naam/ and not /naa/. Where does the /m/ sound come from?

 ำ = aam, it's something you have to learn/memorize

Thank you. I know the vowel  ำ but I didn't see the little circle above the consonant  - I thought there was only a tone mark there. It's so obvious now when I enlarged the text.

"น้ำ is pronounced /naam/"

 

Normally the vowel is short, so /nam/.

In isolation it's long. 

In compounds it's supposed to be short,  but i do know speakers who even in compounds use a long /a/ (they are not native speakers of Central Thai)

I think I've never heard anybody pronounce น้ำ with a short "a". Either there is a rule of which I'm not aware or this is just an irregularity in Thai language.

Just ask any Thai to say ดำน้ำ (diving), the first a is short, the second is long, even though it's  ำ in both syllables.

In น้ำส้ม (orange juice/vinegar etc) the น้ำ sounds short to my ear.

Agree, in ดำน้ำ, the น้ำ sounds long to my ear, as it does when it's on its own น้ำ - water.

2 hours ago, katana said:

In น้ำส้ม (orange juice/vinegar etc) the น้ำ sounds short to my ear.

Agree, in ดำน้ำ, the น้ำ sounds long to my ear, as it does when it's on its own น้ำ - water.

I believe that in a compound word the pronunciation of the vowel is long if น้ำ is the last syllable, otherwise it is short.

น้ำ is normally said long as a single word. Compare อยู่นานไหม and ขายน้ำไหม I don’t think that นำ้ is any longer than ไหม whereas นาน definitely is.