Can't encode and decode live audio in a single gstreamer pipeline

[Ezequiel Garcia]

(This is a repost of a question on stack overflow:
http://stackoverflow.com/questions/34138451/how-to-encode-decode-live-audio-in-a-single-gstreamer-pipeline)

For the sake of testing, I'd like to construct a pipeline that encodes
and then decodes live audio. I have tried with mp3 or aac encoding,
and I can certainly do it if the source is non-live:

    $ gst-launch-1.0 audiotestsrc ! lamemp3enc ! mad ! alsasink
    $ gst-launch-1.0 audiotestsrc ! faac ! audio/mpeg,
stream-format=raw ! faad ! alsasink

In the above cases, the pipeline is constructed and I can hear the
audio playing back. However, if the source is live (e.g.
is-live=TRUE), the pipeline doesn't fail to play, but there's no audio
played back.

Moreover, when the source is live, the pipeline is not blocked (using
log debug level shows the frames are making progress).

This same behavior is found on other live sources, such as alsasrc
from an alsa capture card.

I'm sure I'm missing some key concept, but can't see what!
-- 
Ezequiel García, VanguardiaSur
www.vanguardiasur.com.ar