FFmpeg - Encode AAC
From PiRho Knowledgebase
ffmpeg -i <input.file> -c:a libfdk_aac -ac 2 -b:a 128k <output.file>
These settings target a specific bit rate, with less variation between samples. It gives you greater control over file size, and it is compatible with the HE-AAC profile. As a rule of thumb, for audible transparency, use 64 kBit/s for each channel (so 128 kBit/s for stereo, 384 kBit/s for 5.1 surround sound).
Set the bit rate with the -b:a option.
Reference: [FFmpeg: AAC]
