Wa alaykum salaam,
Technically, it would be acceptable to audit such companies as the audit is not concerned with the promoting, marketing or assisting in sales of unlawful items, rather it is more about reviewing financial statements, internal processes and transaction records to ensure proper recording and reporting.
If you opt not to offer services, that would be out of your caution, prudence, God consciousness and piety, which is commendable and admirable.
Allah knows best