Most people will just find a coach or parent with a good voice and have them record announcements for each player either within the app or with a third party voice recorder and then import the files.
There is also the possibility to have a professional do them... Casey Motter is the stadium announcer for the Atlanta Braves and he has a little side business making announcements for apps like this. His contact info is on Facebook at AllstarIntros. You just give him a call to negotiate a price, send him a screen shot of your team, and he will email back the announcements for you to import. i think he charges around $4 per player.
