So does this apply to voiceovers?
Here is the easy answer: Yes and No.
Fishing for voice work
If you have a full-time job, and unless you work shifts, I wouldn't recommend it, as you won't be available to record during office hours when your clients are available to listen in.
If you have your own home studio, recording the occasional small job (with no client call in) will get you a few quid but nothing major.
If you don't have a full-time job and are working freelance and you have experience as an actor or broadcaster, then yes.
But is it easy money?
Again yes and no.
It's more tiring and time-consuming than you think - and you will need to add the time of travelling to a studio and back.
Dog-proof studio not required
Unless you have a home studio in which case it's not so easy as you have the cost of building and maintaining the studio. Oh and you will need a very quiet space for it.
Not to mention you will need to factor in the time taken to market yourself to get the chance to quote for the voiceover work in the first place.
And then you have to be lucky enough - or skilled enough - to land the voiceover job.