Yes, those sounds are only heard on the PCjr./Tandy 1000 booter versions. They make use of a sound chip which is found in them and not in an IBM PC or compatible.

He said the sounds are present on his XT which i can't really believe
(the tandy/pcjr booters don't run there iirc, and the pc booter should sound
the same as in dosbox).

There are sounds at those points with machine=tandy. I can't say about the booter.

The sounds are most definitely there on the PC/XT. I'll see if I can capture them, and then I'll attach them.

Here are some samples. You'll have to turn the volume up to hear them. I had to open up the PC/XT and practically put the mic on the speaker to record them, so I don't think I can get them any clearer, my apologies.

I tried the booter on my IBM PC Model 5150, and did notice the sounds as recorded. They are so low that I had not really noticed them before.

The sound seems to be virtually the same throughout the instances described. It can also be heard during the almost-omnipresent bird chirping, in between the two distinct chirps.

I am not sure that Sierra intended those sounds to be heard, and if you were in a rather noisy environment or your hearing was less than clear, you may be unable to discern it. Since Sierra developed the game for the PCjr.'s sound chip, these extra sounds may be generated by some overlooked code in the sound routines.

It could very well mean that the physical properties of the actual speaker cone may be generating the sound.

The sounds described are very difficult to make out with all the background hiss and noise in the recordings. I would best describe the sound as something very similar to a telephone using pulse dialing on some phones.

Yes, the sounds are definitely much less audible than the high pitched ones, and record even worse. The telephone pulse comparison is spot on.

have you tried increasing the emulated pcspeaker rate?
This allows us to emulate higher bleeps

I've tried both increasing the rate to 41000 and decreasing the rate to 16000. Both did not work.