The RaceFX help file lists some websites that provide free sound effects files that you can use. You can also do a search engine query to find some other sites.

Yes, but the current version of GrandPrix Race Manager would require you to change the name of the file to the same name of the file that it will replace.

Starting with Version 6.0 of RaceFX and Version 14.0 of GrandPrix Race Manager, these two programs can be interfaced to automatically play sound effects as you run each heat.  See the interface video on GrandPrix Race Manager's Video page, http://grandprix-software-central.com/gprm/videos.php.