Red Hat Linux 9.0 Problem with Internet Via Satellite

Hi,

This is the following station:

A relative is using a Red Hat Linux 9.0 as a FireWall and a Proxy
Server, He have a subscription with a Satellite ISP (2 way internet)
... he have the following problem when you log on the Linux server (as
a user using windows for example) you can browse any website you like
but the following software does not work:
In the clients side you can not access some aplications such as
paltalk, kazaa, yahoo games, and some sites, and Net 2 Phone.
When he closed the firwall completely but the above software will not
work either.
He tried to use Red Hat Linux 8.0 but the same problem is happen with
the above softwares.

Only when he connect the clients (compusters that use windows)
directly to the reciever (satellite), they can open the above
softwares (kazaa and paltlak, etc) but when they go through the proxy
(Red Hat) they can not work !! You can not use the Sound ...

He did tried many times but the same problem happen .... Note about
the IP Address he give a fixed-IP address for the clients PCs ...
Is there any problem is from the Red Hat 9.0 configurations of the
server .. is it a service need to be running or some ports mush be
open.

Again the softwares that does not work are: paltalk, kazaa, net2phone,
yahoo audio

He use .. edhat9 system with squid running as proxy, he installed
linux as server.
and the the following service are running:

apmd, atd, autofs, canna, crond, cups, dhcpd, dhcrelay, gpm, iptables,
isdn, keytable, kudzu, lisa, netfs, network, nfslock, portmap, random,
rawdevices, dhnsd, sendmail, services, sgi_fam, smb, snmpd, snmptrapd,
squid, sshd, syslog, xinetd

Thanks for the help,