The muxer depends on the id3v2 writing code, so link against it.
OBJS-$(CONFIG_OGG_MUXER) += oggenc.o \
vorbiscomment.o
OBJS-$(CONFIG_OMA_DEMUXER) += omadec.o pcm.o oma.o
-OBJS-$(CONFIG_OMA_MUXER) += omaenc.o rawenc.o oma.o
+OBJS-$(CONFIG_OMA_MUXER) += omaenc.o rawenc.o oma.o id3v2enc.o
OBJS-$(CONFIG_PCM_ALAW_DEMUXER) += pcmdec.o pcm.o rawdec.o
OBJS-$(CONFIG_PCM_ALAW_MUXER) += pcmenc.o rawenc.o
OBJS-$(CONFIG_PCM_F32BE_DEMUXER) += pcmdec.o pcm.o rawdec.o