This is a common issue when LW is tooo busy. The media player gets low priority over the other functions. If it happens consistently then disable the media player so your default player opens instead when launching or previewing. Go to Tools>Options>Player & untick option & press apply. But it won't take effect until next time you open LW. If you need a decent player, there's lots around.
Some are 1. QCD, 2. iTunes, 3. Winamp, 4. dBpowerAMP