Let us know the detailed functional parameters/specifications changes between M and MX version of LM386M-1/NOPB(old) and LM386MX-1/NOPB (New) SOIC package.
Within the PDN notification the replacement part LM386MX-1/NOPB) categorized under category Q from the vendor.
Where Q states : Q = Same functionality and PIN-OUT as the compared device but is NOT an exact equivalent.