Convert to WAV using FFMPEG for pipe into LAME?

I'm trying to convert an AAC file into WAV in order to pipe the output into LAME. I'm looking to do this, specifically:

find . -maxdepth 1 -type f -iname "*.m4a" | sort | while read file; do
    ffmpeg -i "$file" -acodec pcm_s16le -ac 2 - | lame -b 256 -m s -q 0 - output.mp3
done

I get the following error:

Unable to find a suitable output format for 'pipe:'
Warning: unsupported audio format

Is there a way to specify the -acodec for the output? Reading the manpage now.

I know that I can convert to MP3 within FFMPEG, but that's not what I want to do ;)