There is no specific Hukam on whether to do Nitnem Paath in Surti or audibly but practically talking, it's best to do Paath audibly, using one's Rasna and Surti. Doing Paath purely in Surti i.e. without the use of any part of the body can be good in high spiritual states but for the most part, we mortal beings should do Paath audibly, using Rasna and Surti.
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥