If the volume will be fixed, why not use the audio-editor and drop the volume that way. To be sure, you give up signal-to-noise (SNR) ratio dB for dB...but for a crossing-gate clanging sound I really don't think you can tell - IMO of course.
Or to lower volume, put a resistor in SERIES with the speaker (not in parallel). That amplifier chip is effectively a voltage-output device so a parallel resistor will simply consume more current with the same voltage (volume) applied to the speaker. If done in series, an 8 Ohm resistor added to an 8 Ohm speaker doubles the resistance load so cuts the current in half. The max voltage out of the amplifier is 5V. So you can do the math to see the Wattage (Watts = V x I) requirement of any resistor. Note that the average power dissipated by the added resistor will be much less - maybe 25% depending on type of sound - than what you calculate for the whatever speaker impedance you are using - you shouldn't need a 5 Watt resistor.
