Da die Frage häufig im Zusammenhang mit Ultraschall und Studio Link gestellt wird, ist die Antwort 48kHz weil der Opus Codec für VoIP/AoIP nur bestimmte Sampleraten unterstützt und 44,1 kHz keine davon ist. Da ein Resampling nur unnötige Latenz bringt und eine Fehlerquelle mehr ist, raten wir die ganze Audio Kette mit 48kHz zu betreiben.
Edit: Hier noch ein Auszug zu dem Thema von den Opus EntwicklerInnen:
How do I use 44.1 kHz or some other sampling rate not directly supported by Opus?
Tools which read or write Opus should inter-operate with other sampling rates by transparently performing sample rate conversion behind the scenes whenever necessary. In particular, software developers should not use Opus Custom for 44.1 kHz support, except in the very specific circumstances outlined above.
Note that it’s generally preferable for a decoder to output at 48kHz, even when you know the original input was 44.1kHz. This is not only because you can skip resampling, but also because many cheaper audio interfaces have poor quality output for 44.1kHz.
The opus-tools package source code contains a small, high quality, high performance, BSD licensed resampler which can be used where resampling is required.
But won’t the resampler hurt the quality? Isn’t it better to use 44.1 kHz directly?
Not really.
If you can’t tolerate the quality degradation caused by a good 44.1 ↔ 48 kHz resampler, then you shouldn’t be using a lossy codec in the first place. Any reasonable resampler will cause far less distortion than even the best lossy codec at its highest bitrate.
Not only that, but even if you could run Opus at 44.1 kHz, it wouldn’t help you. The Opus encoder is heavily tuned for 48 kHz now and using it at 44.1 kHz will cause it to make sub-optimal decisions. This would result in worse quality than a 48 kHz encoding with resampling. If you’ve ever noticed that an MP3 encoder sounds worse at 48 kHz than at 44.1 kHz, then it’s mostly likely not because of the resampling, but because most MP3 encoders are tuned for 44.1 kHz rather than for 48 kHz.