The Voice of Seren is a blessing effect in Prifddinas, moving between clan districts every hour. Two clans will have the Voice active at any given time. While the Voice is active in a clan's district, certain actions related to that clan's signature skills give additional experience or rewards.
The clans that are receiving the Voice are the same in all worlds. Elven clan capes can be rightclicked to tell whether the Voice is currently active in that clan's district. The Master clan cape can be rightclicked to do the same, but will tell the clans districts for which the Voice is currently active in.
When in Prifddinas, two clan icons are visible on the screen depicting the clans that are illuminated by the Voice of Seren. Since 6 July 2015 one can also hover over these icons to see some of the effects for that clan when it is illuminated. Near the lodestone in the Tower of Voices, the player is able to see which districts the voice is active in by looking at the illuminated symbols on the ground. The clan symbols on the ground indicate the direction where each clan district is located.
Inside Prifddinas, the following message is shown when the Voice of Seren changes districts. The colour depends on which district is mentioned.
Clan  Message 

Amlodd  The Voice of Seren is now active in the Amlodd district. 
Cadarn  The Voice of Seren is now active in the Cadarn district. 
Crwys  The Voice of Seren is now active in the Crwys district. 
Hefin  The Voice of Seren is now active in the Hefin district. 
Iorwerth  The Voice of Seren is now active in the Iorwerth district. 
Ithell  The Voice of Seren is now active in the Ithell district. 
Meilyr  The Voice of Seren is now active in the Meilyr district. 
Trahaearn  The Voice of Seren is now active in the Trahaearn district. 
Randomness of the voice
The Voice of Seren rotates in a somewhat random way with some key restrictions. If a Voice is active in a clan, then there will be a two hour cooldown on that clan having the Voice of Seren again. For example, if the Voice of Seren is on Trahaearn from 02:0003:00 then the earliest hour the Voice of Seren can be on Trahaearn again is 05:0006:00.
This means that at the beginning of any hour there are four clans that cannot have the voice on them. Of the remaining four clans that can have the voice, two will be chosen. That being the case, if the Voice of Seren just ended for a clan, there is a 50% chance it will be active for that clan again in exactly two hours.
In this way, if the Voice of Seren just ended for a clan, one can use the 2 hour cooldown to predict the time at which the Voice will be active for that clan again. A good way to check this is with Jagex's Clock on their Twitter page.
After any game update, the Voice can be of any combination, regardless of the voices prior to the update.
Known effects
Icon  Clan  Effects 

N/A  All clans 

Amlodd 
 
Cadarn 
 
Crwys 
 
Hefin 
 
Iorwerth 
 
Ithell 
 
Meilyr 
 
Trahaearn 

Trivia
 An unlisted music track similar to Elven Voice plays throughout the active clans' districts of the city while the Voice is in effect there.
 After the release of Prifddinas, the Voice of Seren gave 20% experience boost for all the clanrelated skills performed under the Voice of Seren. This was reduced for some actions to 5% when the second half of the city was released and turned to the current state on 17 November 2014. On the official forums it was told that this wouldn't happen until the first week of December.^{[1]} This was reverted early in January 2015.
 There are 28 states for the pair of clans for a single given hour (8 choose 2). The possibilities for a given hour depend only on the previous two hours, so the Voice of Seren rotation can be interpreted as a Markov chain with 28 states and order 2. Alternatively, the state space can be defined as the set of possible states for two consecutive hours. The number of such states is 420, or 8 choose 2 times 6 choose 2. In this case, the distribution of states for a certain pair of two consecutive hours, such as 05:00 and 06:00, only depends on the state of "the previous consecutive hours," meaning 04:00 and 05:00. In other words, this is a Markov chain on 420 states with order 1. A Markov chain with order 1 is completely characterized by its transition matrix (whose dimension is n by n, where n is the number of states, 420 in this case). The mixing time (a concept which roughly means rate of stabilization) of an order1 Markov chain is strongly related to the eigenvalues of its transition matrix, in particular, their absolute values. Based on numerical computations and rounding the computed eigenvalues to round values, the 36 largest absolute values of the 420 eigenvalues, in descending order, are exactly 1, $ \sqrt{1/2} $ 14 times, and $ (1/2) $ 21 times. If one stops distinguishing seven of the clans (e.g. considering all clans which aren't Hefin the same), then the 420state order1 Markov chain becomes a 3state order1 Markov chain with absolute eigenvalues 1, $ \sqrt{1/2} $, $ \sqrt{1/2} $. In particular, the secondlargest absolute value of both chains are only $ \sqrt{1/2} $, which suggests that this Markov chain has a surprisingly low mixing time because the clans don't interfere with each other's mixing, and it also suggests a more general fact about Markov chains of this type.