Do you need a modchip to play Sega CD backups

You don't need a mod to play backups.

You do need to make sure that the country code of the game matches that of your console. SCDConv will change the code on most games, but not all.

I believe one of Arakon's multibioses will allow you to play all games.
I have tried changing backup games to PAL using SCDconv to play on my UK Mega CD 1.

But the games suffer from lip synic problems ???
That's because of the 50/60hz timing issue. All NTSC games run at 60hz, or about 17.5 percent faster that their PAL counterparts. Subsiquently, any video that is supposed to be syncronised to an audio track is too slow, hence the sync problems.

To fix this, you need to perform the 50/60hz mod on your MD. Details are outlined in the Miscellaneous section. There is also a topic running in the Genesis section of this forum with the same topic.
Are the 2 second silences at the start of the audio tracks?

If they were binchunked by the original ripper, that may be the case.