Apple : 600 free metro style icons from the cool folks at SyncFusion.
The Noun Project : Hundreds (if not thousands) of icons for just about anything you can think of, all free. These icons will download in Scalable Vector format (SVG) and will need to be converted to PNG at the size of 48 x 48 pixels. Either InkScape or Expression Design can be used to do the conversion.
BRKR Design Icon Pack This pack costs $20 but it includes 350 of the most commonly used icons already sized and ready to go. This single package includes all 350 icons for Windows Phone, iPhone and Android.
Every Stock Photo Search thousands of images, many of which can be used in apps and games. Click on advanced search, then click the Licenses dropdown and check ONLY the boxes for Public Domain and Custom Free. When you find the image you're looking for, be sure to follow the search result back to the original site and read the license to make sure it can be used in your application.
Soundjay Hundreds of sound effects that are licensed for use in apps and games. Most (if not all) of these sound effects should play as-is on Windows Phone. You can use a tool like Audacity to convert any sounds that don't play into ones that do.
Freesound Thousands of free sound effects. Carefully check the license for each sound effect because not all sound effects can be used in commercial applications. Some sound effects may also require you to give attribution (credit) to the author. Not all sound effects on this site can be played on Windows Phone. You can use a tool like Audacity to convert sounds that don't play into ones that do.
Audio Micro has a free music and sounds tab on their home page.
HasGraphics Sprites : 10 sprite sets free for commercial use. Some require attribution, some do not.
Video Game Sprites : Actual sprites from original games, not able to use in commercial games
Lost Garden : Game graphics, asks for acknowledgement.
Open Game Art : Graphics, sprites, icons and sounds. Asks for donations.
CGTextures : Amazing, pretty exhaustive collection of tillable textures that can be used for free in commercial games. Attribution is appreciated but not required.
HasGraphics Textures : Two large tile sets free for commercial use.
Max Textures : Tiled/seamless textures
2D Game Art : for InkScape