323.000 is Boston Center, Sector 09, FL280 and above. The VHF side is 123.875 and the sector is almost always combined with Sector 08, FL270 and below, 135.250/377.100. As you noted this sector controls entry/exit into the Viper Airspace and entry into AR206H/L, it seems on exit the Buffs will often use 377.100 as it's the lower sector frequency, but either UHF could be used. There is a
LiveATC Feed that covers both VHF frequencies were you can hear the controller.
It sounds like towards the end of your clip they were fading out of your range. Can you hear them working Barkeater on 244.500 or the JTAC frequencies 226.900 and 271.200 by any chance ? Maybe too far ?