SlingPlayer for iPad, iPhone or iPod touch
Double-check that the sound in SlingPlayer is adjusted to what you want. Just tap the screen and move the slider at the bottom to the left or right.
This slider also works independently of the Ring/Silent switch on the side of the iPhone. Changing the Ring/Silent switch to Silent won't silence SlingPlayer.
Hardware Volume Control
Check your hardware volume controls of your iPhone or iPad and make sure they are turned up. Note: Ring/Silent mode will not mute SlingPlayer.
Also, if your device has been updated to iOS 4.2 or higher, you can double-click the iOS device Home button to access the system volume while using SlingPlayer. You may need to swipe left or right to get to the volume controls - check out your device documentation for more on how to access these iOS controls. The system volume controls look like this on an iPad:
Note that this technique will disconnect your stream. When it does, just single click the Home button to return to SlingPlayer and reconnect to your Slingbox.