TheVerge reports:

Beats audio has become a pretty familiar brand around these parts: in addition to the ubiquitous headphones, the company has licensed its brand to HP and is majority owned by HTC — both of which manufacture an increasing number of Beats-branded products. Regardless, it's the headphones that built the brand, and the future of who will be manufacturing them is a little more cloudy today as Beats and Monster will not be renewing their contract.
Sources told Business Week that the split happened because of a disagreement over (what else?) money and credit, with each company believing it deserved more of both.

Monster, for its part, is going to move ahead and attempt to (re)build a headphone empire based on the Monster brand. That's the short term plan. The long term plan isn't clear but one fairly obvious move would be to merge with or buy out SOL Republic, a headphone company founded by Kevin Lee who happens to be son of Monster founder Noel Lee. Lee the younger, despite running his competing headphone brand, has said that he would be "honored' to take over the leading role of Monster someday, adding "Cut me open, I’m a monster and will always be a monster." Given Monster's mixed brand image (especially amongst anybody who know about cables), a partnership with SOL Republic would make a lot of sense.

Beats also has a murky future. The company will surely continue to license its brand out, but who will make headphones is unclear. It just so happens to be majority owned by HTC and HTC just so happens to be a manufacturer of consumer electronics. Even if HTC and Beats don't decide to go down that road, you can be sure that given the amount of money and marketshare that Beats has managed to snatch up, they'll find a way to ensure that there are many many more Beats-branded headphones to come.