John 5:37 And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form...
Critics alleges that this verse is contradicted by the many places in the Bible where people hear God speak.
The basic answer here is simple: Jesus is speaking to a specific group of people, not everybody including dead OT figures.
Objection: How do you know his comments were only intended for the people being directly addressed at that time? I could say that the Sermon on the Mount only applied to the people being addressed at that time. How do you know when they apply only to the person or people being addressed as opposed to mankind in general?
The comments in John 5:19-47 are indicated to be an "answer" to "them", and specific reference is made unto those who sent to John the Baptist (v. 33), so that a very small group -- at most, those Jews living at the time who saw John -- can be referred to. Beyond that, let us address this matter of the Sermon on the Mount, etc. by asking this question: When Confucius taught his Analects in a teaching setting, was he speaking just to his class, or to everyone?
The answer is everyone, and that is because in didactic contexts like the Sermon, even though only a small group may be present, it is assumed that the application are universal. Likewise, when an ancient king created laws, and they were announced publicly, it was never assumed that they applied only to those who listened.
The key here is genre, and in that respect, John 5:19-47 is a direct answer to a charge and actions of Jesus' opponents (vv. 16-18). The Sermon, Sinai, and Confucius teaching the Analects are not.