It is easy to record screen & sound of other applications by Display Recorder. Please do the following steps:
Launch Display Recorder.
Press "Record" button (round red button) on the Record screen. Video & audio of your device will be recorded from now.
Switch to the application you want to record. (Press Home and launch that application or Double-press Home and switch to it)
Do anything on that application until you want to stop recording.
Switch to Display Recorder. (Press Home and tap on Display Recorder icon in screen or Double-press Home and switch to Display Recorder)
Press "Stop" button (square black button) on the Record screen. Wait a moment for merging audio & video. The recorded video clip will appear on the "Recorded Items" list soon.
(*) Important Note:
Audio record may be dropped while recording if you run any application that uses system sound. In this cases, audio will be ignore (Display Recorder will inform you), your recorded video will be "Video only".
If you record application that uses OpenGLES, please switch "Capture Method" to "Screen Capture" (Open Settings screen, in the "Video Settings" section, change "Capture Method" to "Screen Capture"). If you keep "Capture Method" as "Direct Access", the video may be black screen. Note: "Direct Access" is faster than "Screen Capture".