Apple's (NASDAQ: AAPL) iTunes Radio is off to a booming start and Pandora Media (NYSE: P) continues to tighten its hold on earbud-donning music buffs. However, streaming darling Spotify has a media event slated for Wednesday, and it could be a doozy.

The Wall Street Journal first reported that Spotify would introduce a free mobile access platform late last week, and a source leaked more details to The Hollywood Reporter on Friday. According to that report, the ad-supported mobile product will let listeners cherry pick the tracks that they want to hear, unlike Pandora and iTunes Radio which build personalized playlists that their algorithms dictate are similar to what you want to hear. If the Spotify playlist is long enough it will just shuffle through those selected tracks. 

Spotify has been a global force in streaming since the Swedish service began expanding internationally. It raised financing last month in a round that values the company at a Pandora-esque $4 billion. More than 6 million of Spotify's 24 million listeners pay as much as $9.99 a month for unlimited ad-free streaming on mobile devices, and that's what makes Spotify so dangerous. There are already far more people paying for Spotify than there are for Pandora. What happens when a limited version of it becomes available to freeloaders?

Pandora and iTunes Radio are very popular. Pandora served up a record 1.49 billion hours of content last month. Apple bragged that iTunes Radio served 1 billion tracks to 20 million users within four to five weeks of its mid-September launch.

As crowded as the market seems to be, the consumer appetite for streaming or premium radio -- I'd coin this market as streamium if Phillips didn't already have an audio component with the same name -- is still growing.

Popular satellite radio monopoly Sirius XM (NASDAQ: SIRI)  has grown its base of paying subscribers every quarter for more than three years. Even Sirius XM threw its hat into the ring with a stand-alone streaming platform that went on to add several Pandora-like elements.

However, despite Pandora, Sirius XM, and iTunes Radio amassing nearly 110 million active accounts between the three platforms, there has historically been room for growth. The smartphone and connected automotive markets are growing nicely, widening the audience of potential listeners.

Investors should still pay close attention to what Spotify announces on Wednesday. If it changes the game, some listeners may be changing their tunes.

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Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends Apple and Pandora Media. The Motley Fool owns shares of Apple and Sirius XM Radio. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.