That is such a wrong reading of the story. Like, in another galaxy wrong.
- Rabbi Eliezer did not “hear voices in his head.” The voice was heard by all present (and the preceding miracles were witnessed by all present), and in fact the other rabbis responded to it.
- The other Rabbis did not regard Rabbi Eliezer as insane. (I supposed this follows from point one.) They simply argued that the divine voice (post-Moses) (as well as the ability to invoke miraculous occurrences) has no place in the process of deciding religious law.
- The notion the the Rabbis would have believed that hearing the voice of G-d privately means one is insane would imply that they think the same of the Biblical-era prophets, and from the fact that their statements in the Talmud are all based on the recorded words of these prophets, it is self-evident that they did not believe this overall.