生成節拍器音聲文件

這些音聲是由tools/gen-metronome.sh（需要 Java）生成的。它只生成WAV；這裡用ffmpeg轉成mp3。不然，自己想辦法。節拍生成器的使用方法如下：

tools$ gen-metronome.sh 
Usage: gen-metronome.sh outFile bpm dur beat
  where: outFile is the output WAV file name.
         bpm     is an integer for beats-per-minute.
         dur     is duration; can end with 's' (seconds) or 'm' (minutes).
                 Further, if ends with '_', the last 10 beats will be softer.
         beat    can be a file name, or a note frequency such as 440.
A few examples:
    gen-metronome.sh metron-120-5m.wav    120 5m  ../images/metrobeat.wav
or: gen-metronome.sh metronA4-125-5m_.wav 125 5m_ 440
or: gen-metronome.sh metronA4-125-30.wav  125 30  440
or: gen-metronome.sh metronA3-140-30.wav  140 30s 220

For your quick reference, the frequencies of musical notes are:
  C4=261.6 D4=293.7 E4=329.6 F4=349.2 G4=392 A4=440 B4=493.9