Hmm, I probably shouldn’t tell you 
about the Welsh word for law, act or statute, which is deddf. And I definitely shouldn’t tell you it’s a feminine noun, so you would say y ddeddf for “the Law”. 
Or perhaps that would help you! 
Now that’s just plain evil. But then again, I’ve seen a couple of tongue twisters in Welsh that are worse. “Llwy a chyllell”, for example. And when I couldn’t remember this one, and tried Googling “tongue twisters in Welsh” to find it, I saw more hunllefau. 
Its kind of weird. In some instances, like a lot of songs, I can tell which sound is F and which is DD. Then I turn on one of SSiW challenges, and in certain words, the DDs sound like Fs practically every time. 
Apparently I can only hear sounds correctly when I’m not trying to learn new words. 
Then I turn on one of SSiW challenges, and in certain words, the DDs sound like Fs practically every time
There are unavoidably so many components between speaker’s voice and the listener’s brain in delivering the challenges that it is not surprising that certain sounds are lost or even changed. Here are some of them
Recording microphone quality (important)
Recording software
Transmission to Cloud
Reception from Cloud to listener’s device
Listeners’s software
Listener’s earphones/headphones/speakers (important)
Even if all these components are perfect (which they never are), you still have the path between the listener’s ear and brain which depends on things like ear wax and age (the frequency range audible to wrinklies like me is much less than in our prime 
)
I could offer an account of the acoustic spectra of the “f” and “dd” sounds in Welsh but it wouldn’t add a lot. 
In the meantime, you may find the word lists helpful until you can have a head to head conversation with a native speaker free of technology (which, I admit, may be a problem in the States. )
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I look at the word list after the first time I’ve run through a challenge, so I can know how words are spelled and be sure that I haven’t misheard anything. And once, I couldn’t figure out a Welsh word (I think it was “ddiddorol”) even after hearing it multiple times, so I cheated and typed the English equivalent into Google Translate to check it. Sometimes sounds just escape you. And as a bonus, if you think it’s one way, sometimes you start to hear it that way because it’s what you expect to hear.
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sometimes you start to hear it that way because it’s what you expect to hear.
That is certainly true and is an additional “component” in the issue of reproducing and transmitting sounds.
If you can say “ddiddorol”, rather than “vivorol”, it may help to say it over and over to “teach” your brain what to expect. It should be an “interesting” exercise anyway. 
I’ve moved these posts to their own thread since the perception and pronunciation of sounds such as “dd” and “ll” crop up regularly, in the hope that we can collate helpful advice.
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Since @Garys mentioned deddf, and @meowmocha enjoys tongue twisters 
, I can suggest a special training with the Datblygu song “Dim Deddf, Dim Eiddo”. 
I could post link and lyrics and translation, but with the warning that there’s a few words/expressions that could be labeled “explicit” or beeped in some contexts, not sure if they’re ok for everybody here! 
In general, although I still find it difficult to tell DD from F, from my experience I would confirm what @HuwJones says about components between speaker’s voice and listener’s ears and brain - and luckily speaking to people in person is way better than through all kinds of devices!
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