Too many to even think about!
As a music fanatic from as long as i can remember, i am only too aware that there are billions of hours of fabulous music out there that i will simply never hear before I die.
It strikes me when I recall how i heard a particular sound that I now love, and the sheer serendipity of the moment.
I was at Download a few years ago, and wandered into the press arena where a guy was mixing 12" singles for the chillin' press types. I heard the Rolling Stones' 'Miss You' remixed by Dr. Dre and fell in love with it instantly.
https://www.youtube.com/watch?v=6nJn4x0Yhj8
If I had not wandered in there just at that time, I would have missed it, and who knows what he played before, and after, that I may have loved even more!
You have to accept that some wonderful things you find later rather than sooner, and enjoy them and carry on catching up - that's the wonderful bit.
The 'lost' sounds are just that - lost, so don't dwell on them, chase new music every day - I do!