I have some speech files in MP3 format that are rather 'flat'. Is it possible to re-record them and boost the treble content?
They will eventually be converted to WAV, so the eventual file size isn't critical. I don't have any audio Players having treble and bass adjustments.

If the record level (content) is sufficient, you could but hiss comes into play quickly. Don't expect much and you'll be ok. I just made some stereo cassettes from micro cassettes from a pocket recorder and without any EQ at all, just putting the volume up to Dolby levels sounded great!

I don't recall if MusicMatch Jukebox (http://www.musicmatch.com/) has a graphic equalizer or not. (There's a limited ability free version if interested) It's been awhile since last using it. Windows media Player has one on the output so this wouldn't be of much help unless you are mixing them down to cassette or similar. (Pc to pc would work great!)

You could google a search for mixers and graphic eq's and mix down with one but most trials I've found online are like 'one time use only' and after that the features end up greying out on you. :(

If you can play your MP3's on a stand alone player (ie. Walkman), you could put an EQ in between that and your pc's line in and record through anything you have for software including 'Sound Recorder'. At home I use 10 band EQ's, but have 15 and 31 band EQ's available if I need. But all that's needed to clear up some voice recordings is simple bass/treble controls. With midrange as a bonus and parametric midrange a treat!

As said earlier, just putting the level up to max with 'Sound Recorder' could work out fine without any other outside help. Sound Recorder has an analog meter that is VERY sensitive to distortion if you go too far. It may take a few takes but it may be all that's needed for your MP3's.

Edit again: Err, converted MP3's that is... :o (and not only analog either) /Edit

My son pointed out that Real Player has an equaliser, but there's only 3 settings. I gave it a try, but it does expose things that make it sound worse instead of better. WinAmp has a more comprehensive one, but I removed WinAmp as it started playing up.
The MP3s in question are radio broadcasts @32Kb/s - not the best setting. Despite what we're told, 32Kb/s isn't really that good for speech files.