By default yes — 48 kHz AIFF stays 48 kHz in AAC. If you need to downsample for compatibility (96 kHz → 44.1 kHz for CD burning), the sample-rate option applies high-quality resampling in the same encode pass.
Same-codec re-mux: 10 to 30 seconds. Re-encode to a different codec: typically 10 to 20% of source duration, so a 1-hour AIFF → AAC finishes in 6 to 12 minutes. Batch jobs parallelize across workers for further speedups.
Why is the AAC louder or quieter than the AIFF source?
AIFF (Formata Pelê Veguhastina Dengê) formatek pelê dengî ya nekompresandî ye ku bi gelemperî di hilberîna deng û muzîkê ya profesyonel de tê bikar anîn.