SUBDIRS := $(wildcard 0*/)

all : $(SUBDIRS)

guide: guide.wav
guide.wav: $(addsuffix guide.wav,$(SUBDIRS))
	sox --combine concatenate $(sort $^) $@

$(SUBDIRS) :
	$(MAKE) -C $@

%/guide.wav:
	$(MAKE) -C $* guide

.PHONY : all $(SUBDIRS)