as salam alaikum
The Qur'an says: "Whenever you are greeted with salutation, greet with better salutation or return the same. Verily Allah takes account of everything" (4:86).
A believer should answer the greeting of another believer in a way that the salutation is heard, otherwise cannot be defined as "salutation". There is no sin if the other person do not say or hear it unintentionally. However the greeting of peace "as salam alaikum", and returning the greeting, is one of the beautiful aspect of Islam, it is considered a du'a and strengthens brotherhood among the believers; it is better therefore to make sure that it always reaches its destination.
With prayers for your success.