I use a voice recorder to record lectures classes. The quality turns good but, I would like to improve it much more.
I would like to raise the voice of speaker, turn down buzzing sounds making it clearer. Is there any advanced software I could use?

Firstly, does your recorder record to a cassette tape (analog) or to digital? If the latter, what is the format of the resulting recording (e.g., mp3)?

You could possibly use an audio editor (like Adobe Audition or the freeware Audacity) to filter the unwanted frequencies outside the range of human speech, which might get rid of some of the buzzing sounds you describe.

Perhaps someone else here at HA knows how do do this a much simpler way.

I use digital. The format is Sony digital voice file. But it can be turned to WAV and mp3. I will try out the softwares, thanks!

What is your equipment and how do you have it set up?

Digital Voice RecorderICD-SX46

I sit in large lecture hall with 350 students. Mostly I sit in the back seats. Lecturer speaks with a microphone so my recorder catches the sound clear. I record in stereo mode with *.dvf format.

I will add an external Mic to it soon.



Sony ECM-ZS90 Switchable Stereo Microphone with Super Directional Mode

I'm not sure what .dvf is, but my guess is that it's a proprietary format based on CELP. (Lossy vocodec.)

I think the only way to do filtering would be to either filter on playback (if the player supports DSP effects) or to transcode/compress after applying filtering.

The first might be tricky if you must use a Sony player to play the files on your computer, and the second may involve compression artifacts/size increase/both.

I once had a Sony ECM-MS907 and found the quite good at recording from a specific direction. It might be a better choice.