Does ffmpeg's aac encoder need a audio bitrate?

It totally depends on the AAC encoder you are using. ffmpeg can use several AAC encoders:

• aac (ffmpeg-internal) – defaults to 128 kBit/s for both mono and stereo.
• libfdk-aac – defaults to 128 kBit/s for stereo and 96 kBit/s CBR for mono.

In general, you should choose a higher bitrate than the default for aac. This is because it produces somewhat lower quality than libfdk-aac at the same bitrate. Do that by setting -b:a appropriately, e.g. -b:a 192k.

I tried to summarize the VBR options on my homepage, and the FFmpeg Wiki has good info on AAC encoding as well as some example commands.

Also, when you see a line similar to this in ffmpeg's log output:

Stream #0:0: Audio: aac (libfdk_aac) (mp4a / 0x6134706D), … 96 kb/s

Then the kb/s shown here do not necessarily reflect what the encoder will really use, as it's up to the specific implementation.

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There are also some encoders that are not supported anymore by ffmpeg:

• libfaac – defaults to VBR setting of 100, which results in about 128 kBit/s for stereo audio (reference).
• libvo-aacenc – defaults to 128 kBit/s for both mono and stereo.
• libaacplus