How to judge a narrator in five minutes
Play the sample twice: once at normal speed, once at 1.25x. A narrator who still sounds natural sped up will stay comfortable across ten hours; one who blurs is usually over-performing.
Listen for the breath. Audible inhales between every line of dialogue become exhausting on a long drive. Then check a dialogue-heavy chapter, not the prologue — prologues are the most rehearsed part of any recording.
Finally, ask whether the narrator is acting the characters or describing them. Both work, but a full-cast style performance in a quiet literary novel will fight the writing.