Indeed, the source contains a section that explicitly changes "default" to "hw:0" with a big /* HACK HACK HACK */ comment above it. Almost seems like there might have been a reason for this, but I haven't been able to find out what that was yet (might just have been alsalib being silly and not accepting 'default' in old times; seems to work fine these days, who knows).

        * ext/alsa/gstalsa.c: (gst_alsa_detect_rates),
        (gst_alsa_detect_channels), (gst_alsa_probe_supported_formats):
        Small code cleanup.

        * ext/alsa/gstalsamixer.c: (gst_alsa_mixer_open),
        (gst_alsa_mixer_new):
        Remove hack that always set the device to hw:0*.
        Properly find the card name for whatever device was configured.
        Do some better debugging.
        Fixes #350784.

        * ext/alsa/gstalsamixerelement.c:
        (gst_alsa_mixer_element_set_property),
        (gst_alsa_mixer_element_change_state):
        Cleanups.
        Handle setting of a NULL device name better.