I don't think they'd call it a digital
voice recorder if it was capable of making a satisfactory recording of a rock concert with
any external mic. The mic input would probably easily overload at a rock concert since it was not designed for recording music. Besides I think it only records at a bitrate of 128 kbps, which is also a big limitation.
If you want to make recordings that sound good for less than $300, I would suggest the following. Ditch the voice recorder. Pick up a Sharp Minidisc recorder with a mic in and line in from E-bay which can regularly had for $100 or often far less. For a mic, contact Chris Church (Church-Audio) about getting one of his STC-11 cardiods, which go for less than $125 shipped. This is a great mic for the money and is not very sensitive (which is good for recording loud concerts). The Sharps have very good mic preamps, so you should be able to record shows that are quite loud mic in without distorting. If you even do run into a problem with distortion, that can be solved by later buying a battery box and going line in.
Chris is a member of this group. You can use the link below to send him a personal message:
http://taperssection.com/index.php?action=pm;sa=send;u=14500The only annoyance is that you would have to make analog transfers of your recordings to your computer in real time, but the recording would sound way better than anything you could pull with the Olympus. To avoid this you would need to be willing to spend considerably more money on a recorder. Such as over $300 for an Edirol R-09 plus maybe $40 or more for a 4 gig memory card. A used iHP-120 for $150 or so is another possibility, but in a lot of situations they need a preamp to make a quality recording. An iHP-120 might do alright by itself for loud rock concerts though, since you wouldn't need to turn up the gain too much. Maybe someone else who has used one without a preamp can chime in on this.
Of course there's no reason you couldn't buy just the Church-Audio mic first and try it with your Olympus. The Church-Audio mic should be much less likely to distort when recording loud sounds than the low cost Sound Pro's mics since it has a lower sensitivity. But I really doubt that any mic would make good recordings with the Olympus.