INPUT := $(wildcard serial-s01-e??.mp3)
OUT75 := $(patsubst %.mp3,%_1.75.mp3,$(INPUT))
OUT50 := $(patsubst %.mp3,%_1.50.mp3,$(INPUT))

all: 50 75

clean:
	rm -rf serial-s01-e??_*.mp3

50: $(OUT50)

%_1.50.mp3 : %.mp3
	/usr/bin/sox --no-show-progress $< $@ tempo 1.50

75: $(OUT75)

%_1.75.mp3 : %.mp3
	/usr/bin/sox --no-show-progress $< $@ tempo 1.75