Webm to mp4 conversion with ffmpeg

I can’t seem to get webm to mp4 conversion to work on my VPS. My PHP scripts work fine on my local machine with Mac OS X El Capitan. Even when I do it from the command line I get errors:

/usr/bin/ffmpeg -i a8c53f18-b304-42fe-9bdb-2833b3814c5b.webm video.mp4

That produces these errors:

ffmpeg version 0.8.16-4:0.8.16-0ubuntu0.12.04.1, Copyright © 2000-2014 the Libav developers
built on Sep 16 2014 18:33:49 with gcc 4.6.3
The ffmpeg program is only provided for script compatibility and will be removed
in a future release. It has been deprecated in the Libav project to allow for
incompatible command line syntax improvements in its replacement called avconv
(see Changelog for details). Please use avconv instead.
[matroska,webm @ 0x254e9c0] Unknown entry 0x56AA
[matroska,webm @ 0x254e9c0] Unknown entry 0x56BB
[matroska,webm @ 0x254e9c0] Unknown/unsupported CodecID A_OPUS.
[matroska,webm @ 0x254e9c0] decoding for stream 0 failed
[matroska,webm @ 0x254e9c0] Estimating duration from bitrate, this may be inaccurate
Input #0, matroska,webm, from ‘a8c53f18-b304-42fe-9bdb-2833b3814c5b.webm’:
Duration: 00:00:11.40, start: 0.000000, bitrate: N/A
Stream #0.0(eng): Audio: [0][0][0][0] / 0x0000, 48000 Hz, 1 channels (default)
Stream #0.1(eng): Video: vp8, yuv420p, 640x480, PAR 1:1 DAR 4:3, 1k tbr, 1k tbn, 1k tbc (default)
[buffer @ 0x25e5940] w:640 h:480 pixfmt:yuv420p
Incompatible sample format ‘(null)’ for codec ‘libvo_aacenc’, auto-selecting format 's16’
Output #0, mp4, to ‘video.mp4’:
Stream #0.0(eng): Video: mpeg4, yuv420p, 640x480 [PAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 90k tbn, 1k tbc (default)
Stream #0.1(eng): Audio: libvo_aacenc, 48000 Hz, 1 channels, s16, 200 kb/s (default)
Stream mapping:
Stream #0.1 -> #0.0
Stream #0.0 -> #0.1
Decoder (codec id 0) not found for input stream #0.0

If I read the error message correctly, avconv is compiled without the support for the audio stream in the webm video you have there. Input stream 0.0 is

 Stream #0.0(eng): Audio: [0][0][0][0] / 0x0000, 48000 Hz, 1 channels (default)

and before that avconv complains saying

 [matroska,webm @ 0x254e9c0] Unknown/unsupported CodecID A_OPUS.

Thanks, but what does that mean exactly? I don’t have much control over how these videos get encoded. They’re coming from Tokbox.