Jane's absence was explained in a letter saying that she was visiting relatives in England. Maureen O'Sullivan was sick of playing Jane, but her absence probably had more to do with the fact that the franchise had switched from MGM to RKO, and O'Sullivan was an MGM contract player.
M-G-M was unwilling to let RKO use their recording of Johnny Weissmuller's signature "Tarzan yell," which had accompanied the character as he swung through the jungle clinging to vines in every Metro Tarzan film. The one heard here is a much shorter, less robust rendition, and clearly not the original version.
When M-G-M sold the rights for Tarzan to RKO, Johnny Weissmuller and Johnny Sheffield were part of the package. Maureen O'Sullivan was still under a long term M-G-M contract, and was not so secretly delighted that she would no longer be obligated to play Jane, a role that, by this point, she openly loathed.
In her role as "Zandra," Frances Gifford was essentially auditioning to replace Maureen O'Sullivan as Tarzan's love interest. RKO was apparently unimpressed by the chemistry between Gifford and Weissmuller, since she never again appeared in a Tarzan film.
Sig Ruman (this film's bumbling German sergeant) experienced a career resurgence during World War II, frequently cast as a comic relief Nazi like the one he plays here.