Tvheadend

Use "master"
or download manualy
https://dl.bintray.com/tvheadend/ubuntu/4.1-343~g41e0a26/pool/main/t/tvheadend/

Your first batch of errors is because you've most likely run the echo deb... command twice. If you look at /etc/apt/sources.list then you should see these duplicates, and you can just comment out or delete the extra copies.

Your second one seems more fundamental. It's telling you that you need a later version of libc (which is a core library of C calls) than you actually have installed. However, upgrading this is probably a bad idea as it's central to so many things: if a later version loses (deprecates) some calls, for example, then the rest of the operating system might break. Or might not. Or might one day, when you're not looking

Now, while that may help with the diagnosis, I have no idea of the cure. I don't know if you can build tvh locally against earlier libc, for example, or whether there's a later release of OMV that has more recent libraries? 2.13-38+deb7u8 was Debian Wheezy, while Jessie moves to 2.19-18 (ref: https://packages.debian.org/search?keywords=libc6-dev), so it depends on what version of Debian your release of OMV is based upon...

Okay first error is solved. that wars easy

OpenMediaVault 2.1 (Stone burner) \n \l
Copyright (C) 2009-2015 by Volker Theile. All rights reserved.

this is my installed version.
Distributor ID: Debian
Description: Debian GNU/Linux 7.8 (wheezy)
Release: 7.8
Codename: wheezy

maybe this helps?

Those ubuntu pkgs don't work on OMV. At least, not until the OMV project upgrades to Jessie. Perhaps they don't upgrade OMV base distro since Debian switched to systemd.

I would recommend you contact OMV project maintainers for clarification on that. e.g. if they have plans to move to jessie in future. IRC Freenode.net #openmediavault

Sorry, we can't change those 'ubuntu' pkgs to support jessie - it's simply too old now.

Have problems installing tvheadend after adding the key and the entry to sources.list
Also checked that there is only one available entry in sources.list
Also tried to put the stable main entry with same result

echo deb https://dl.bintray.com/tvheadend/ubuntu testing main | sudo tee -a /etc/apt/sources.list

Everything seems fine from adding key, source, and running apt-get update

However when I try to install I get this:

root@P5KR:/etc/apt# apt-get install tvheadend
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
tvheadend : Depends: libdvbcsa1 (>= 1.1.0) but it is not installable
Depends: liburiparser1 (>= 0.6.0) but it is not installable
Recommends: xmltv-util but it is not installable
E: Unable to correct problems, you have held broken packages.

Tried adding the old repository and were able to install 3.4 fine.
Anyone that can help me on the way?
It would be highly appreciated.

A little update on this.
Never figured out the problem, so scratched my ubuntu and reinstalled the whole thing since it was just a testing installation anyway.
Worked right away so something odd was going on.
It might be worthwhile to make a fresh install instead of banging your head against the wall due to weird problems.

Had some weird stuff going on with tvheadend earlier too, deleting the config files and starting over made the trick.