I've heard sources where he was said to have voices speaking to him and somewhat insane, is there any truth to this?
To my knowledge, the only source for this legend of Nebuchadnezzar as having a mental illness is the Bible, specifically the book of Daniel, Chapter 5. I am not sure what modern historians think about using the Bible as a historical document, but I know I wouldn't use it to support claims made about a character depicted as an adversary of the Hebrew people.
As the other posters here have noted, the Biblical references to Nebuchadnezzar's mental imbalance appear to be garbled references to the last Babylonian king Nabonidus; whether or not Nabonidus was actually imbalanced he was certainly somewhat of a religious eccentric or at least an innovator who was portrayed in a negative light by the Babylonian priesthood for his changes as is recorded in for example the Nabonidus Chronicle. In particular he was accused of sponsoring the cult of the moon god Sin at Ur and especially Harran at the expense of the traditional Babylonian cult of Marduk. But again this is as much a matter of the efforts of his political enemies to delegitimize him.