It's not a long vowel (武内 =
take - no¹ - uchi), so generally you'd pronounce the vowels separately:
no - u. A couple examples of Japanese folks saying it:
[1] [2].
Depending on the speaker, though, the vowel sounds might get merged together or even turned into a long
o, like here:
[3].
1: The no particle is sometimes omitted from place/personal names when written, even if it's still pronounced.