I say Sow-knee, so does everyone I've met.
Except my mum, probably the only person in the world that pronounces Sony different to everyone else.
She insists on saying Soh-knee with a short o, "because it's japanese" (nevermind that she's never actually learnt japanese and was only there once in 1967 when she got her Spotmatic).

(I've also tried pointing out that Sony actually has a Latin-root 'Sonus' (Sow-nus) meaning 'Sound', but that won't change her mind either)