As stated earlier in the thread, regardless of what type of learner you are, ~90% of the US med schools out there record their lectures, which are usually better quality than anything you could record yourself (some even record audio and visual); however, quality probably does vary by school.
If it would help you learn and you need it now, then by all means get a recorder now; otherwise, wait and see whether or not your school records the lectures (no reason to waste money).
As a side note, while I do not disagree that some people learn better by listening, you cannot solely practice this method of studying in medical school. It would behoove you to explore alternatives or simply be aware that the day will come where you have to modify you study habits. There are a couple reasons I bring this up. First, there isn't enough time in the day to re-listen to every lecture more than once; especially if you attended the original lecture (even one repeat would be a chore). Second, there is too much information to learn outside of lecture, such as in board preparation. You may be lucky to find audio versions of review books or their associated lectures online (some of these are awesome), but only a few review books actually have audio versions. Third, as you get into the clinical years of medical school (classically 3rd and 4th year) and beyond, lecture time is greatly reduced. Much of your learning comes from reading about your patients and their conditions while you take care of them (no audio files here). And the didactic sessions that you do have in the clinical years, are being transitioned to case based discussions (or problem based learning groups) all across the nation; audio recordings may be useful here, but trying to re-listen to them after 60-80 hr/wk in the hospital will be the last thing you want to do.
I'm not trying to discourage anyone, but adjustments will need to be made to your study habits. My class has ~200 students and I don't know a single person who didn't have to at least moderately change their study style.
Those of you in medical school probably know what I am talking about; please correct me if your observations differ. Those of you who are pre-med, you'll find out soon enough.
-senior medical student (19 days left) / admissions committee interviewer