In actuality a cheap set of headphones in a quiet listening enviroment (e.g.; no background noise) will reveal artifacts of transcoded audio files, it doesn't take an audiophile to notice them. The original lossy format has already thrown quality away, therefore transcoding to another format can introduce more artifacts. The only way to combat this is to use a lossless audio format (Wav, WavPack, Flac, etc) and then encode to a lossy audio format (MP3, MPC, OGG, etc).
To make claims of not hearing a difference you really have to conduct a blind listening test to determine if really you can or can't actually hear a difference.
@ Eldmannen
I also avoid WMA hence there's the chance of enabling DMA which will make the files useless on other systems and devices. However, OGG isn't supported in all devices therefore your suggestion won't in any way help oli.
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For those whom want to know about audio formats, codecs, transcoding, etc., I'd recommend learning on a forum I've been a member of for a few years called Hydrogenaudio, there's a wealth of information there that has tought me allot about audio.
I did state that I am in no way an audiophile, so therefore I was just giving my opinion, which is the opinion of a regular guy.
Also I dont know about in the USA, but from my observations of people in the UK, headphones are generally used in places with excess noise, such as commuting to or from work using buses, trains, tubes, walking, or whatever other means is available. These are, I believe, the times when people would want to listen to music through headphones, rather than using other means at times when other means are available, which would probably then remove the need to listen to WMA's anyway.
And besides all of this I was just trying to help oli achieve what he was trying to achieve.
Whether this was the right or wrong approach will always be up for argument, as many things always are, but oli seemed happy.
I just wanted to point out about transcoding because if he deleted his original files in favour of the original source mp3's I didn't want him upset in the future or years down the road if and/or when he may have noticed a degradation of quality.