Where can I watch Seiyuu shows?
Hanazawa Kana's Hanazawa Kana Hitori de Dekiru kana:
You can listen to this radio program on Thursday, 23:00-23:30 Japan time, by visiting the following URL: https://www.uniqueradio.jp/form/form5.php.
This channel is legal under Japanese law.
I don't know if it's accessible from outside Japan.
You will be asked in Japanese where you live, your occupation, and your address. There is no option to choose an address outside of Japan, but you may choose Tokyo.

The Onsen (Japanese) is another well-known channel that offers voice actor's programs, but I don't know if it's accessible from outside of Japan.
At least the following YouTube channels are legal under Japanese domestic law. As these are YouTube channels, they can be accessed from outside of Japan.
These are provided by the holder of the official content.

Dengeki Bunko Channel
TrySail official YouTube channel
Second Shot Channel
Phonon Channel
YOUDEAL LIVE Channel
GA Bunko Channel
GAGAGAch!!!
warnerbrosanime
ANIPLEX
Hi★Channel
Bushiload
NBC Universal Anime/Music
KING AMUSEMENT CREATIVE
Anime Channel by FuRyu

For some content it seems to be possible to use Youtube's automatic subtitle generation.
Some of above content providers are American companies, but the contents are related to Japanimation.
Some channels seem to have special sub-channels. For example, the NBC Universal's channel, has the Gochiusa's channel as a sub-channel.　The operation of such a subchannel may be limited to when the program they want to promote (like Gochiusa) are on the air.