I have so FLAC files at 48 Khz (DAT) and I want convert them to 44.1 Khz 320 cbr Mp3. My question is if this would have any negative effects since the Khz won't be the same as the source file.
No, LAME (which I presume you are using, as you mentioned dBpowerAMP in your other thread) is perfectly able to resample with effectively zero probability that you or anyone else could hear a difference.
Edit: Of course, some might say that there’s no need to resample if you don’t have to! But it doesn’t matter much either way, as resampling’s impact upon the audio is trivial compared to the remainder (i.e. the MP3-/lossy-specific aspects) of the processing. Which you probably won’t notice either
Yep, you shouldn't have to resample. I think every MP3 player on any platform or device I've had can playback 32, 44.1 and 48KHz sample rates for MP3
What I would do is to simply encode the file at 48kHz, to avoid having that extra unnecessary processing done to the audio.
Something else to think about is that most computer sound cards operate natively at 48 kHz, or a multiple thereof. (Not sure about portables and car stereos.) So 44.1 kHz audio gets resampled on playback to the sound card's native 48 kHz. So, changing your file to 44.1 might actually cause it to be resampled twice - 48 to 44.1 before encoding, then again back to 48 on playback.
I consider myself blessed when I come across 48 kHz audio for this reason. Not only do I get marginally higher resolution, but I can avoid it being resampled on playback on my PC.
However, I'll end by seconding what db1989 said, that is: More than likely, you won't be able to hear a difference either way. So this choice has little effect in the end. Personally I'd avoid resampling simply because it's an unnecessary step.