Voiceover requires a financial investment in a studio and training, as well as plenty of time spent learning and actually recording your project. Therefore, it is usually not free. Note that, I will consider recording your project at a discount, if you are a nonprofit organization that I identify with.
However, I did not just say "free voiceover" for nothing. Here you can find free voiceover that you can download and use for your personal enjoyment. Commercial use is not allowed.
If you have an idea for free audio, contact me. If I like your idea, I may implement it! To get some ideas, listen to my voice demos.
You can add the following audio files to the various tabs of your website, so that each time someone visits a tab, the relevant file plays automatically. For example, when someone visits the Welcome tab, the Welcome audio file will play.
To download a file, right-click it, then click "Save File As" or "Save Link As". You can then install these audio files using your site authoring software.