Beats is a brand for a separate company. HTC pays, in one way or another, for that brand.
If you get a GE ROM, you'll get beats. If you go pure AOSP, google does not pay for the beats brand, and thus does not include it by default.
Being that, in software, Beats is really just an EQ..You can emulate it with DSP manager or flash a number of beats zips floating around for AOSP.
If you want Beats-like sound in AOSP, just open DSP manager, and make the EQ look like a smiley face. Bring the lows up, and the highs up, leave the middle mostly the same. Adjust to taste, and you'll have, more or less, the software side of beats.