I think this has been quite interesting.
Thing is, as others have picked up on, I have not really been great at letting DH parent in his own way (although thinking about it, it's very helpfully similar to my way) so when it comes to little niggly things like this I never know whether it's fair for me to "suggest" (!!) he changes things (hence my first foray into AIBU).
Thinking about it now, it does seem a daft thing to really put my foot down about anyway. I mean, how many minutes of DS's life are we talking about here?
BUT, having said that, I think in any case it's more what was underlying DH's headphone thingy that worried me ie: am I the only one thinking about these things (speech development, socialisation, etc). I think a lot of mothers have that head-doing feeling that the buck always stops with them, no matter how involved or brilliant their DP/DH is. My DH definitely seems to have this sheen of "yeah, just a dad now, cool, tra la la" and it perhaps hides how much he is really thinking about DS and I should give him more credit. And, yes, learn from him and relax a bit myself!
I think there would be other areas that I'd see as more clean cut - like if DH always had the TV on around DS, never read him a book, never played with him, etc. Still would rather he didn't put his headphones in, either, but I reckon he'll, ahem, grow out of it in a few months anyway as DS becomes more toddlerish.
Thank you all for your comments.