Are there guides at the major sites in each city? Do they have audio guides?
You really need to specify what sights you're interested in. We may not all agree on what is "major".
For Barcelona: La Sagrada Familia, Casa Mila/La Pedrera, Casa Batllo and the Sant Pau modernista site all have audio guides. I don't remember about Parc Guell, the Picasso Museum, the Miro Museum or the MNAC. It's likely they have them. You'll probably find information on the ticketing section of each website.
You need advance tickets for La Sagrada Familia, Casa Mila, Casa Batllo, Parc Guell and the Picasso Museum; if you try to walk up, you'll be in an incredibly long ticket line and may well not be able to buy a ticket for that day.
Still in Barcelona: The Palau de la Musica Catalana has a facility-provided tour. The English tours can sell out, though I don't think that happens very far in advance. This is another one for which you should get your ticket before you arrive at the sight.
I have not been to Tarragona.