ffmpeg capture audio in raw format
Use sound.wav as your output, instead of sound.mp3.  Or if you literally mean raw PCM data with no headers, not just uncompressed, then try

ffmpeg -f dshow -t 10 -i audio="virtual-audio-capturer" -f s16le -y "sound.pcm"


Play it back with

ffplay -channels 2 -f s16le -i sound.pcm


Unless you really need headerless, I'd suggest WAV files for ease of use.  Sample format, rate, and number of channels stored in the file is a Good Thing.