"W3C", the Organization which is responsible for the recommendation concerning html has recommended the ogg/vorbis as the format for sound files.
Alternative Browser as Firefox, Google-Chrome and Opera full fill to this requirement.
A great advantage is that there are no fees for using this format which is fully free, is developed by an non commercial organization and don't include any patents.
Commercial vendors as Microsoft and Apple don't support this recommended format. They support, instead, the largely used mp3 format. Mp3 is subject to license fees and therefore don't comply with mit the "W3C" rules.
The official reason is that there have doubt about the fact that ogg is not covered by patents and that some strange patent companies may require important fees. An other aspect is probably that it is possible to disallow to exchange the sound files since there can be bound to a particular device.
Opera a little commercial company and firefox a non commercial organization are not able to license such formats as mp3 and so on.
As producer of a slide show with embedded sound, xou may also be asked to pay fees if the sound is submitted as mp3 file.
The supplier for alternative browser full fill the "W3C" recommandations and provide there browser for almost all operating systems. There are also a lot of freely available tools which allows to convert or produce ogg sound files.
Google-Chrome support as well ogg as mp3 but is not available for all OS and in particular BSD based OS.
Therefore the choice for the sound file format shall be ogg