My 85 yr old mom has 1 yr raspy, weak voice. History post nasal drip. What are some causes of this 'lose' of voice? Losenges seem to help a little.

Dysphonia. Voice changes can be associated with colds, injuries and surgery among other things. It's only been a short while so, she should be resting the voice along with hydrating herself. If it persists beyond the 2 week mark, get your mom to an ENT or laryngologist so that the doctor can look at her voice with a 'scope'.
Get scoped. Your mother could have simple vocal nodules or scar tissue or something more serious. She needs to get this looked and soon. Chronic heartburn, often at night while sleeping can also cause this but get it checked out to be sure.