Not long ago, the Gran Teatro La Fenice eliminated daily group tours and replaced them with audio-guided ones. However, it looks like group tours are back as well, basta to call HelloVenezia at +39 041 2424 in advance to get information on when tours might be available in your language. They are not daily, so at this writing, calling them (just select English) is the only way to reserve your place. You may be able to stop by a HelloVenezia location as well, if you’re already in town.
If you prefer an ad-hoc visit, the audio-guided tour lasts 45 minutes, and costs €7 per person, and is available any time the theatre itself is (see below). To get the most out of it, you could review the History of the Theatre section offered on the Fenice web site (in English) before you attend.
Please note: Although it’s no longer necessary to reserve, you must still verify the opening hours, which will vary depending on the season and events currently taking place in the theatre. Once again, this is easily done at HelloVenezia +39 041 2424 (English spoken), also a source for ACTV public transit information, the Biennale, the Fenice season and other cultural events, the Venicecard, and more.