FFMPEG: AC3 to AAC conversion issues

Ok, this one has me scratching my head - and being a n00b isn't helping.

In short, I had a script which I had perfected to covert MKV video to MP4.

However, my Ubuntu 10.10 box bit the dust and installed 11.04.

However, my script is now having problems with the following line:

ffmpeg -i "${title}".ac3 -acodec libfaac -ab 576k "${title}".aac

When it reaches the relevant stage in the script, it returns the error:

Unknown encoder 'libfaac'

I have tried swapping libfaac for aac, which seems to be installed/recognised by ffmpeg - however that codec is just way too slow.

I have no idea how to either;

1. get libfaac back on my Ubuntu 11.04 install, or
2. find a better codec than AAC to convert the audio.

Any help to point me in the right direction is greatly appreciated.