The easiest way to get you started would be to record them with
Audacity and save them as a WAV or MP3 file.
The downside to this is that you will have to do it in real time so if your lecture lasts 1 hour, it will take 1 hour to record.
I use this method for many things. If you go on to a site that has ringtones for example that you can preview but have to pay to download simply use Audacity to record the preview and you have it for free.
