Even if it's not really obvious, I hear payions. But honestly, if the sentence is true, we never say that in French, we can write it but when we speak we prefer to say "On payait tous les mois" to avoid the confusion with "Nous payons tous les mois" because even for a native speaker it's hard to hear the difference.
Geminated sounds are very hard to hear, especially semi-vowels such as [j].
- payons - /pɛ.jɔ̃/ or /pe.jɔ̃/
- payions - /pɛj.jɔ̃/ or /pej.jɔ̃/
English rarely distinguishes between a single consonant and a geminated consonant (all the examples I've found for English are disputed). There is almost no difference between /j/ and /j.j/, and I can't hear it here, which doesn't mean it's not there as English speakers are not used to distinguishing geminated sounds from non-geminated. I can't even hear the difference when I know I'm saying it differently, unless I insert a gap between the two syllables.
Does that mean you are saying "we paid every month" is correct? I have answered my own question next time it came up, and it is correct. So I looked up the difference between paid and payed. They are quite different. https://writingexplained.org/payed-or-paid-difference