Unfortunately, there isn't currently a way to differentiate which $network the notified nick is supposed to be affiliated with. You can obviously script a solution for it, the simplest way is to add the network as the Note: field and then check for that on NOTIFY; or you could keep track of that information in a hash table/ini file/whatever else.
[Notify]
DALnet.Nick1=Note
DALnet.Nick2=Note
EFnet.Nick3=Note
on *:NOTIFY: if ($readini(MyNotify.ini, Notify, $+($network, ., $nick))) .splay NotifySound.wav
on *:NOTIFY: if ($hget(Notify, $+($network, ., $nick))) .splay NotifySound.wav
on *:NOTIFY: if ($istok($notify($nick).note, $network, 32)) .splay NotifySound.wav
To get the last example to work, you'd need to go through every nick in your notify list (in Alt-B / Notify) and use the Note: field, adding each network you want to watch for that nick separated by a space.
