Nrg are disk images created by Nero Burning ROM. You can have nrg images converted to audio without burning any disk.

First install AnyBurn freeware and convert the nrg image to bin.

AnyBurn creates a bin image and a cue file containing information on audio tracks.

Get XRecode and load the cue file. XRecode recognises both the bin image and its track information in the cue file offering you to select the destination type (mp3, wav, etc).

Select the audio format that you like, convert, and enjoy!