I want to download some audio tracks and have multiple settings to choose from. Main settings are frequency (44.1, 48 or 96 kHz) and bitdepth (16, 24 bits). I will use audio as background music for films on DVD and Blu-Ray video disks. How will this affect end result if I download audio in 48kHz/24bits or 96kHz/24bits format and then transcode it to 48kHz/16bits format (which is most common for DVD and Blu-Ray as I understand)? Audio is in WAV files and for editing I use Premiere Pro (Adobe Encoder).
The general rule of thumb is that you would want the original files at the highest sample rate and bit depth that you can get. That way, if they need to be processed - equalization, compression, delay, reverb, etc. - then you are doing that at a high rate and the end result will be better when you down-sample to your desired final sample rate and bit depth.
That said, if you are typically just laying in music underneath dialog or narration without a lot of processing, you could easily just download the files as 16-bit, 48kHz to begin with. As you said, for video editing and DVD, 16-bit, 48kHz is the standard.