I can confirm something similar on Debian. Using your examples of tones at a sample rate of 8 kHz, the second tone is not played at all if it is about 150 ms or less in duration.

Since sound() is just a wrapper around the audioplayer class, here is an example that shows the same behavior for me using audioplayer.play directly:

fs = 8000;
y = [cos(2 * pi * 440 * (0:0.2 * fs) / fs), cos(2 * pi * 880 * (0:0.125 * fs) / fs)];
p = audioplayer (y, fs, 8);
play (p)

The truncation may be in the way Octave is feeding data to portaudio or it may be in the portaudio library or in a lower layer. It should be possible to debug the portaudio interaction in audiodevinfo.cc to see what conditions cause the audio stream to stop, and whether Octave needs to make an additional call to flush remaining data for example.

The sound function seems to stop the sound playback too early. This shows up espacially with short sounds below one second. I guess that the sound length is shortened by around 200 milliseconds.

Attached there is a small function for creating configurable beep patterns. It is suitable for testing this behavior. See inline documentation for usage.

The following call should play two short beeps a lower and a higher one. The second beep in the example is not played on my windows system:

beepcode([700,100;1400,200])

When adding a time of silence to the end of the beep sequence the second beep is played depending on the length of silence:

beepcode([700,100;1400,200;0,50])
beepcode([700,100;1400,200;0,100])
beepcode([700,100;1400,200;0,150])
beepcode([700,100;1400,200;0,200])
beepcode([700,100;1400,200;0,300])