Tom Baker has tentatively agreed to do Big Finish audios and Nicholas Briggs working on several scripts to send to him. I think the first one is done or close to it. Reportedly, Baker has been bugging Briggs about seeing the script. Plus Sladen and Jameson have expressed interest in working with Baker again.
What's your source for this? None of the news pages are reporting it. There was a report about 6 months ago that Tom Baker was in discussions with Big Finish, but it was quashed because Tom was committed to BBC Audio. Actually, most of what's here seems to be taken from that early report. The last official word is nothing has been arranged or discussed formally. However as Baker has broken the ice with the two BBC Audio arcs he's done so far, it does mean he's open to doing more. But if BBC Audio wants him for a third arc, then he won't likely be doing Big Finish.
And that's why. She has a TV series. There's no need for her to do any audios for Big Finish. And as for audio dramas she's recording ones for BBC Audio based upon The Sarah Jane Adventures. As noted, if BF gets Tom Baker to come on board, then she'd do it. Otherwise, why bother? The whole idea of the Companion Chronicles was to reintroduce companions previously unfeatured in Big Finish in their original forms - Romana I, Jo Grant, Ian Chesterton, Zoe Heriot, Sara Kingdom. More recently they've expanded this a bit to include more commonly heard companions such as Evelyn Smythe, Peri Brown and Benny Summerfield. Sarah Jane was already well covered with her own series, and with the TV series even more so.If that's the case, why haven't they done any Companion Chronicles with her? They did an ongoing series called Sarah Jane (which was before the Companion Chronicles started) prior to the show, but once the show started, Sladen stopped doing anything with Big Finish.
That said, a more likely scenario is a BBC Audio story arc featuring Sarah Jane and the Doctor, IMO.
One sign to watch for might be if Richard Franklin does a Companion Chronicle as Mike Yates. If he does (and he's been conspicuous by his absence in the series), then that might mean BBC Audio is loosening the strings a bit.
Alex