During Mozart's lifetime, would an Austrian peasant be likely to hear Mozart's music?

Sort of. The Austrian parish school system focused heavily on religion and music (an account of Haydn's childhood states he left this system when it became clear to his parents that his abilities were beyond what was available to a wheelwright's son). So a peasant could well have heard music by Mozart played in the provinces, played by local musicians, given that sheet music was increasingly available in the 1780-90s. (See Melton here: http://www.journals.uchicago.edu/doi/full/10.1086/422930)

If you're asking whether a peasant could attend one of Mozart's operas in Vienna, this is more unlikely, but not impossible. The tickets for the fourth tier of the Burghtheater (where Il Seraglio, Figaro and Cosi fan tutte all premiered) were 20-24kr. each. The end of serfdom in 1781 would have allowed some movement between Vienna and its hinterlands. I've not been able to find a peasant's average wage for the region, but in 1779 a mason's average daily wage was 15kr. (via International Institute of Social History, found here: http://www.iisg.nl/hpw/data.php). So it's not outside the bounds of reason to suggest that a peasant could have saved up for a ticket, and had the mobility to attend.

(Nb- my background is in British theatre of the 18th/19th century, so there may be elements I've missed as the social context differs.)