今天需要把aac转mp3,刚好有人整合了一下，搬运过来。
转载自ffmpeg audio format conversions

1. Audio

1.1. WAV - Waveform Audio File Format

1.1.1. wav to mp3

Convert wav to mp3 with ffmpeg:

ffmpeg -i audio.wav -acodec libmp3lame audio.mp3

1.1.2. wav to ogg

Convert wav to ogg with ffmpeg:

ffmpeg -i audio.wav  -acodec libvorbis audio.ogg

1.1.3. wav to aac

Convert wav to acc with ffmpeg:

ffmpeg -i audio.wav  -acodec libfaac audio.aac

1.1.4. wav to ac3

Convert wav to ac3 with ffmpeg:

ffmpeg -i audio.wav -acodec ac3 audio.mp3

1.2. OGG - Free, open standard container

1.2.1. ogg to mp3

Convert ogg to mp3 with ffmpeg:

ffmpeg -i audio.ogg -acodec libmp3lame audio.mp3

####1.2.2. ogg to wav
Convert ogg to wav with ffmpeg:

ffmpeg -i audio.ogg audio.wav

1.2.3. ogg to aac

Convert ogg to aac with ffmpeg:

ffmpeg -i audio.ogg  -acodec libfaac audio.aac

1.2.4. ogg to ac3

Convert ogg to ac3 with ffmpeg:

ffmpeg -i audio.ogg -acodec ac3 audio.ac3

1.3. AC3 - Acoustic Coder 3

1.3.1. ac3 to mp3

Convert ac3 to mp3 with ffmpeg:

ffmpeg -i audio.ac3 -acodec libmp3lame audio.mp3

1.3.2. ac3 to wav

Convert ac3 to wav with ffmpeg:

ffmpeg -i audio.ac3 audio.wav

1.3.3. ac3 to aac

Convert ac3 to aac with ffmpeg:

ffmpeg -i audio.ac3  -acodec libfaac audio.aac

1.3.4. ac3 to ogg

Convert ac3 to ogg with ffmpeg:

ffmpeg -i audio.ac3 -acodec libvorbis audio.ogg

1.4. AAC - Advanced Audio Coding

1.4.1. aac to mp3

Convert aac to mp3 with ffmpeg:

ffmpeg -i audio.aac -acodec libmp3lame audio.mp3

1.4.2. aac to wav

Convert aac to wav with ffmpeg:

ffmpeg -i audio.aac audio.wav

1.4.3. aac to ac3

Convert aac to ac3 with ffmpeg:

ffmpeg -i audio.aac  -acodec ac3 audio.ac3

1.4.4. aac to ogg

Convert aac to ogg with ffmpeg:

ffmpeg -i audio.aac -libvorbis audio.ogg