Problem solved!
Found a simple and clean solution. No glitches anymore, even with the most demanding flac or HD video.
This is, one should add to the end of the line in /boot/cmdline.txt the config smsc95xx.turbo_mode=N only. Have a clean sound now.
From Raspberry Pi and realtime, low-latency audio [Linux-Sound],
It seems that this mode is supposed to increase speed performance of USB and Ethernet, when working together, by reducing the number of packets exchanged between the core and the smsc95, by using some encapsulation, by putting USB together with Eth packets through their USB connection.
Although USB audio requires real-time service, rather then vary fast data-rates. By doing this, one loses real-time capabilities, and get the glitches in the audio, due to real-time deadline misses.