An ubuntu-desktop  machine(call it A) outputs RTP traffic to 224.0.0.56

It is able to see its own traffic:
A:~$ sudo tcpdump -n net 224.0.0.0/8 -c 10
[sudo] password for vegar:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
22:43:33.254962 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.267653 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.267673 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.281077 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.281102 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.293753 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.293770 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.312635 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.312702 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:43:33.319869 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
10 packets captured
12 packets received by filter
0 packets dropped by kernel

Another ubuntu-desktop machine(B) can see the same traffic pattern(and can play back the audio):
B:~$ sudo tcpdump -n net 224.0.0.0/8 -c 10 -i wlan0
[sudo] password for vegar:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on wlan0, link-type EN10MB (Ethernet), capture size 96 bytes
22:39:49.704724 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.705388 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.718085 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.718771 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.730973 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.744882 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.756907 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.757606 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.770266 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
22:39:49.770945 IP 10.0.1.142.56485 > 224.0.0.56.46788: UDP, length 1292
10 packets captured
10 packets received by filter
0 packets dropped by kernel

However, the ubuntu-server machine(C) sees this:
:~$ sudo tcpdump -n net 224.0.0.0/8 -c 10
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
22:40:27.334291 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:32.333530 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:37.332805 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:42.332062 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:47.331331 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:52.330599 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:40:57.329858 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:41:02.329125 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:41:07.328388 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
22:41:12.327657 IP 10.0.1.142.50500 > 224.0.0.56.9875: UDP, length 220
10 packets captured
10 packets received by filter
0 packets dropped by kernel