NRen2k5 & Cori,
You're right--I was able to record the mono tracks without problem (using SonicStage 2.1). My earlier problems burning these tracks must have been related to the problems I was having overall, not due to the fact that they were mono recordings. I was mislead by one of the Sony knowledgebase articles that indicated that older MP3s ripped in mono might not be able to be transferred to the MD player. (A reinstallation based on the instructions on this site, rather than the info provided by Sony, helped me accomplish a clean reinstallation and seems to have solved all my problems--at least for now.)
Thanks for the help!!
Bill