[tebasuna51]

Audio don't have FramesPerSecond then you can't change the FPS to the audio.

Maybe you need change the duration of the audio. What is the duration of both mkv's?
If is the same maybe you don't need recode the audio.
If is different seems you need recode the audio.

With eac3to you can slowdown the german audio and encode it to the desired bitrate (not sample rate)

eac3to german.ac3 output.ac3 -slowdown -448

Or you can use ffmpeg, equivalent to eac3to changing the pitch (and supossing the samplerate is 48000):

ffmpeg -drc_scale 0 -i "german.ac3" -af "aresample=50050, asetrate=48000" -acodec ac3 -center_mixlev 0.707 -ab 448k "output.ac3"

Or, if you want preserve the pitch:

ffmpeg -drc_scale 0 -i "german.ac3" -af "atempo=0.959041" -acodec ac3 -center_mixlev 0.707 -ab 448k "output.ac3"

@Cary Knoop
With sox you need decode to wav and after recode to ac3, ffmpeg can do all at same time, and I think use the same 'tempo' routines than sox.