A few years ago, Sirius XM Radio (SIRI -1.59%) was the butt of investing jokes.

No one's laughing anymore.

Shares of the satellite radio provider hit a fresh five-year high of $3.85 today. Even with the media darling's ambitious share repurchase efforts, Sirius XM has never been more valuable than it is right now.

These are the kind of lofty heights that would woo naysayers, but they're surprisingly silent, too.

Exchange data released on Friday shows that Sirius XM had 303.2 million shares sold short. That's a big number -- and it is -- but it's actually the lowest tally since November of last year. Short interest peaked at 414 million at the end of February.

The ideal mix of buoyant car sales and an improving economy has been working wonders for Sirius XM. It has tacked on nearly 1.2 million net additions through the first half of the year, and the average subscriber is paying more than before as last year's price increase kicks in. Boosting the value of its premium streaming add-on platform is also helping.

There was always the fear that Liberty Media (FWONA) would spin off or unload its stake in Sirius XM after achieving its controlling interest, but that's a rarely voiced concern these days. Sirius XM is a workhorse. Why would Liberty Media want to stop riding?

It also only helps Sirius XM that it keeps growing its subscriber base in a seemingly changing marketplace. Pandora (P) continues to grow at a faster clip than Sirius XM -- and streaming apps continue to pop up -- but Sirius XM just keeps adding to its rolls.

If anything, one might even argue that Sirius XM and premium streaming services including Spotify and Pandora One have whetted musical appetites of the middle class and better. Radio is more valuable now than it was in the terrestrial radio gravy days where playlists were narrow and limited by ad blocks.

Sirius XM may never accelerate its growth to the levels that we're seeing in some of the online names, but as it tests its pricing elasticity on a growing base of radio buffs, it's hard to call for a ceiling.

Sirius XM is at a five-year right now, and it won't be a surprise to be there again before too long.