What does the audacity project .aup record

System Win XP sp3. Installed audacity via .exe. I am a new user. It took a little time but I found no problem in making a recording from a high quality cassette recorder. The cassette was recorded from an elderly LP. I reduced the clicks and amplified the sound as per the manual. It sounded OK on basic PC speakers but with periodic slight bass boomy noises. I opened the .aup file and assumed that the click removal and sound amplification would still be incorporated. However, after I applied the high pass filter and then exported as a wav file the boominess has gone but the recording is rather feeble so I had to screw the amp volume up. Am I wrong therefore in assuming that the .aup file saves any modifications such as click reduction etc? Does it only save the basic original recording?
Thank you in advance to anyone who can help me on this.

The .AUP file saves the current state of the Audacity Project.
There is no audio data in an AUP file (see here: http://manual.audacityteam.org/o/man/audacity_projects.html)

Thank you for trying to help. What you say might literally be true. However, if you open an .aup file you get what I presume is an instruction set which operates on the saved music file and when you click file then export you get a modified music file. Hence in my case if this is correct, I would get the clicks reduced and the overall sound amplified. Then, if I open the .aup file again and apply the high pass filter I would get this effect in addition to the previous two modifications. If this is correct my observation is that the second saved .wav file has less amplitude than the first as found in a listening test to the CD. Perhaps this is merely because the bass has been reduced leaving the rest untouched so that the overall level of sound is less. I also found the default high pass filter too strong as the resultant sound was a bit ‘tinny’ . My lesson is to use a rewriteable CD next time so I can check the sound before permanent recording because my PC audio system is not good enough on playback to really know what the result is.

That seems like a reasonable deduction.

That is frequently a problem with PC audio systems. PC speakers are usually rubbish. I use a Behringer UCA 202 USB sound card (about $25) to connect my laptop to my main stereo system and that gives much better sound.