The only advice is to record the sound at the volume you want to have it played back on the N900.

I use aplay in a script that plays clock chimes every 15 minutes. It turns out that aplay's volume-setting attributes were never implemented, so it plays a file at whatever volume it's recorded at. I had to use Audacity to reduce the recorded volume of the audio files very low, as aplay just blasts them out regardless of what the N900's speaker volume is set at.