Probably yes, just an audio file.
I've been hiring voice actors for my projects and I can think of two reasons why someone would like to ask you to record specific samples before hiring.
1. The freelancer's portfolio doesn't cover the style needed for the client's purpose. Perhaps the job requires some acting and the samples only demonstrate narration with a standard voice (no acting).
2. The client only needs one small thing to be recorded in real, posts a job that appears big, asks for s short custom-made sample and gets it, and never starts a contract because what was needed was done for free by a freelancer who is new to this platform.
You must assessment yourself if any of the above matches your case. You haven't provided enough background that would help me to recommend you the best course of action.
