Generally, it’s best to follow any recommendations given by the colleges to which you’re applying. But if the guidance is limited, and if you have a relatively newer smartphone with a good recording ability, then yes, I think it’s okay to use it. Just make sure the sound quality is good – here are some guidelines for a college audition video, which would apply even if you’re not planning on being a music major:
You might want to post this in the Music Major forum, as questions about music supplement recordings come up quite often over there, although they are mostly for music majors sending in their prescreen audition videos.
I just realized that I didn’t address your question about whether it’s ok to send an audio, instead of video recording. I Since you are performing in classical voice, an audio might be ok if you feel that your expressive ability does not add to the quality of the performance. Otherwise, I think a video recording is best.