Hopefully, the two following sentences are grammatically valid:
旅行の後で帰った時話を話す。
旅行の後で帰った時話が話される。
If so, can I use either as a basis for a relative clause modifying 話; are the two following sentences valid? Do they differ in meaning?
土産話は旅行の後で帰った時話す話です。
土産話は旅行の後で帰った時話される話です。
What I mean to say is "Souvenir stories are stories told after coming back from a trip". Does either sentence convey that meaning, or are they both wrong altogether?
Your sentences have several flaws unrelated to your main concern. 話を話す ("to talk a talk"?) is weird, and you have to say 話をする instead. 旅行の後で帰る sounds clumsy and redundant, and it's better to say 旅行から帰る. When you introduce the definition of a word, it's better to use とは instead of は.
After fixing these, yes, you can say all of the following.
- 旅行から帰った時話をする。
- 旅行から帰った時話がされる。
- 土産話とは、旅行から帰った時する話です。
- 土産話とは、旅行から帰った時される話です。
する話 and される話 effectively refer to the same thing in this case because people can automatically find the implicit subject that makes the most sense. This question may be related.
