"British People will dance to music they can sing along to wheras
Americans will go with a groove I have used this to great affect and
its true"
WoT ?!
I guess it depends what kinda British people ur talking about and what kind of American people.
Perhaps a one brush statement defining how British and American people respectively enjoy music is utterly useless.
- as for the OP - there is no one answer.
Also - I'm not aware that the music sync in any Apple commercials are particularly memorable, creative or cool.
The main change RE: the strength of music within a marketing message - I've seen over the last decade is this, and it's something for you to think about in you dissertation perhaps.
The Ad' industry now faces new challenges and has been in decline for a short while. However the music industry has been facing much tougher challenges for much longer. Over the last decade music rights holders have been willing to license published, well known, famous music for much (much) less - this has the effect of allowing creative teams to get a bit lazy .. with the famous track often way outshining the spot .. (and often detracting from the message) ..
All in all there can be no set % of the responsibility of the eyes & ears .. more the two should work appropriately & in harmony .. one should not steal or detract from the other ..