Hello! I submitted a proposal yesterday, and today wanted to add a voice sample (as requested by the client), however it is saying that I cannot resubmit because the client has denied my proposal. How can I contact them to include this sample? Thanks!
At this point, I don't think you can. If they had opened an interview with you, you would be able to contact them through the message function to add anything else they might request. It sounds like they didn't do so, and most likely chose another vendor.
I wouldn't spend any more time worrying about it. Just remember to add a sample next time you submit a proposal.
Looking over your profile, versus what you're saying in the original post, it sounds like you're trying to get a VO gig. My guess is, even if you'd added a voice sample, you probably wouldn't have gotten it, because there's nothing at all in your freelancer profile that suggests you have any experience or interest in doing them. If you want to add Voice Over as a skill, you'd need to completely redo your profile in order to add the relevant information, but honestly, I think that would only muddy the waters for you.
(If you WEREN'T trying to get a voice-over gig, the request for a voice sample seems ... weird.)