Answered by Shaykh Rami Nsour
Question: Is my prayer sound if I missed the first two rakats of Isha prayer with the Imam, then recited them quietly when I got up to finish my prayer?
Answer: If one stands up to make up the first two units (rak’as) of an audible prayer, then they would have to recite loud enough to where the person standing next to them can hear them.
If a person is in an area where others are praying, then one should only recite loud enough to hear themselves. This is so as not to disturb others who are praying.
If a person does not recite audibly unintentionally when they should have, then they prostrate after the prayer (ba’dy) twice. If they left audible recitation intentionally or due to ignorance of the ruling, then nothing is owed and the prayer is sound.
For ladies, regardless of the situation, they are to recite audible prayers only to where they can hear themselves.
[Mukhtasar Khalil]
source: http://seekershub.org/ans-blog/2014/05/02/on-arriving-late-to-audible-congregational-prayer-maliki/