Recording with Audacity 1.3.3

Operating system: win98SE
Sound card: Audiophile 2496
I am doing some voice recording using a condenser microphone and a mixer.
I’ve tried both Audacity 1.2.6 and 1.3.3. What I noticed about 1.3.3 is that it drastically decreases recording dynamics (or even cuts off) frequencies over 18kHz, which doesn’t happen in case of Audacity 1.2.6.
The recording parameters are 24 bitrate and 48000 Hz and the recording device is set the same - PCM In 1/2 Delta AP - in both cases.
Is it connected with some adjustments that I’ve got wrong or it is an in-built limitation?

Krzysztof

BTW the noise reduction in Audacity 1.3.3 is splendid!

The problem is that with Audacity 1.2.6 you can export the recording as uncompressed PCM “.wav” only with 16 bitrate at maximum (at least under win98SE, but as far as I know this is also the case with XP - I’m not sure about Vista).
The way round the problem is to do the recording in Audacity 1.2.6 and save it as a native Audacity project, then reopen it in Audacity 1.3.3 (Audacity 1.3.3 will claim the format from Audacity 1.2.6 is not supported but in most cases it will open it) and finally export the project as PCM “.wav” with 24 bitrate.
Audacity 1.3.3 has much wider possibilities of exporting than Audacity 1.2.6.