That's a minimum voltage rating on the PCB. You can exceed it as much as you wish, as long as the cap will physically fit. Higher rated caps are physically bigger, as @Nic says above.
That make sense using the phone volume on the input.
I may not have been clear about my intentions on the output. I will not add an auxiliary on the output, just the ability to plug cheapo earphones if that's all I have available at the moment... wife sometime steals my good headphones.
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