In default.php, this line:
oneBit = new OneBit("/plugins/PlayMP3Links/1bit.swf");
had to be modified. Because my forum's URL starts with "/forum", the URL for 1bit.swf had to be
oneBit = new OneBit("/forum/plugins/PlayMP3Links/1bit.swf");
Now the plugin works. Perhaps a more flexible code fix will be able to handle this for everyone, but here's a workaround.