Emusic.com, the pioneer among online music sites, has been sold, and, more importantly, is scaling back its service. It’s still the only service offering downloads in the standard MP3 format, still has the best selection of independent labels, but is now owned by Dimensional Associates LLC, a private equity group, but has limited its main subscriber base, paying $10 a month, to no more than 40 downloads a month.Now, those wanting more — up to 300 tracks per month (approximately 25 albums) — will have to pay a monthly charge of $50. Sad, but inevitable. And it’s still cheaper than most online music services.