The nūn in إِنَّ carries shaddah. A mushaddad nūn has an audible, measured ghunnah. This is its own rule, rather than simply the result of a preceding nūn-sākinah combination.
Do not compress it to something like «inna» with no nasal hold, or exaggerate the hold.
Compare «إِنَّ» and «إِنْ». In the second phrase, the ruling of the sākin nūn depends on what follows.
Opening interactive controls...
Ghunnah of nūn mushaddadah
Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf
The nūn in إِنَّ carries shaddah. A mushaddad nūn has an audible, measured ghunnah. This is its own rule, rather than simply the result of a preceding nūn-sākinah combination.
Do not compress it to something like «inna» with no nasal hold, or exaggerate the hold.
Compare «إِنَّ» and «إِنْ». In the second phrase, the ruling of the sākin nūn depends on what follows.