cd into

/usr/ports/audio/linux-c7-alsa-plugins-oss

uninstall it

make deinstall

then issue

make

while it configures, interrupt it with ctrl+c

then cd into /usr/ports/audio/linux-c7-alsa-plugins-oss/work/alsa-plugins-1.1.6

save below as patch

diff --git a/oss/ctl_oss.c b/oss/ctl_oss.c
index b8847b1..b05ba18 100644
--- a/oss/ctl_oss.c
+++ b/oss/ctl_oss.c
@@ -52,7 +52,7 @@ static const char *const vol_devices[SOUND_MIXER_NRDEVICES] = {
    [SOUND_MIXER_CD] =  "CD Playback Volume",
    [SOUND_MIXER_IMIX] =    "Monitor Mix Playback Volume",
    [SOUND_MIXER_ALTPCM] =  "Headphone Playback Volume",
-   [SOUND_MIXER_RECLEV] =  "Capture Volume",
+   [SOUND_MIXER_RECLEV] =  "Master Capture Volume",
    [SOUND_MIXER_IGAIN] =   "Capture Volume",
    [SOUND_MIXER_OGAIN] =   "Playback Volume",
    [SOUND_MIXER_LINE1] =   "Aux Playback Volume",

then apply it under /usr/ports/audio/linux-c7-alsa-plugins-oss/work/alsa-plugins-1.1.6

patch -p1 < patch


Hmm...  Looks like a unified diff to me...
The text leading up to this was:
--------------------------
|diff --git a/oss/ctl_oss.c b/oss/ctl_oss.c
|index b8847b1..b05ba18 100644
|--- a/oss/ctl_oss.c
|+++ b/oss/ctl_oss.c
--------------------------
Patching file oss/ctl_oss.c using Plan A...
Hunk #1 succeeded at 52.
done

then cd back to /usr/ports/audio/linux-c7-alsa-plugins-oss

make install clean

now you can chroot into /compat/linux and run amixer and alsamixer  both will work.