If you use Marble, you can record your own voice and use that. For the recording part, see http://community.kde.org/Marble/VoiceOfMarble (translations and recording subpages). When you're done and have the .ogg files, place them in /home/user/MyDocs/.local/share/marble/audio/speakers/nicholes/ (create this directory on the N900). After starting Marble, click on the routing infobox (four large icons on the bottom) for some seconds and choose "Configure" in the upcoming menu. There you'll be able to choose nicholes as speaker then.
If you succeed, it would be lovely if you could describe the steps at http://userbase.kde.org/Marble/Custo..._Custom_Voices. Thanks