avconv can do a video fade out, but doesn't appear to fade out audio. I'd recommend you use sox instead. The syntax is:
sox song.mp3 faded.mp3 fade 5 240 8
In this example, song.mp3
is your source, faded.mp3
is the output, 5
is the fade-in time (you can leave that at zero if you only need a fade out), 240
is the length of the audio, and 8
is the fade-out time. So, for your specific example, you would use:
sox SPEX_pilot_02.mp3 preteach-words.mp3 fade 0 25 4