There are two main places I usually check for defaults:
/frameworks/base/packages/SettingsProvider/res/values/defaults.xml
and:
/frameworks/base/core/res/res/values/config.xml
Its also worth checking the device directory for overlays which just override the original files:
/device/<company-name>/<product-name>/overlay/frameworks/base/core/res/res/values/config.xml
Default wallpaper usually hardcoded to:
/frameworks/base/core/res/res/drawable-nodpi/default_wallpaper.jpg
Default volume levels for various streams(int[] DEFAULT_STREAM_VOLUME):
/frameworks/base/media/java/android/media/AudioSystem.java