[how do you play an avi file?]

I go with VLC player.

[How to convert iTunes DRM protected music to MP3 AAC AIFF WAV on Mac]

AppleMacSoft DRM Converter uses a CD-RW and iTunes itself to convert all of your DRM music files to standard audio format files (MP3, AAC, Apple Lossless, AIFF, WAV).

[Wav to mp3 Converter]

I recommend FlexiMusic WaveEditor, a simple audio/Mp3 editing software, to edit and converting music.

[Music composing software]

I use Fleximusic Composer, a multi-track program for composing, editing and mixing of music using sampled instrument sounds.You can create your own songs and tunes at home easily, by mixing various music pieces and samples of your choice. It is also available for free download.

[Sound related question]

Use one of the three small buttons (Blue/White buttons in the bottom) or use the menu > Select > Select both channels to toggle the selection to select only one channel. Go to Menu >File>Format > Change to Stereo, 16 bit, 22050 Hz. (For voice recording you have set the format to 16 bit, 22050hz. For Music recording set the format to 16 bit 44100hz.). Before recording select the channel. If you want left channel click the left side part (top) or if you want Right channels click the right side part (bottom), or use the small button to toggle the selection. Click record button to record the voice. Click "Recording Source and Volume" button. Record control window will appear, in that Choose "Microphone".Use Start, Stop and Done buttons for recording. After finishing recording in one channel, you can toggle/select the other channel and record in that.